cli: add --characters-file

[m4rc1e]

This PR allows users to use a file containing a regex string to filter out characters.

@madig could you please test this (don't have a win machine to hand)? Here's you test string with the required mods.

[!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\\\]\^_`abcdefghijklmnopqrstuvwxyz\{\|\}~ ¡¢£¥§¨©ª«®¯°²³´¶·¸¹º»¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿĀāĂ㥹ĆćĊċČčĎďĐđĒēĖėĘęĚěĞğĠġĢģĦħĪīĮįİıIJijĶķĹĺĻļĽľĿŁłŃńŅņŇňŊŋŌōŐőŒœŔŕŖŗŘřŚśŞşŠšŢţŤťŪūŬŭŮůŰűŲųŴŵŶŷŸŹźŻżŽžǍȘșȚțȷˆˇ˘˙˚˛˜˝ẀẁẂẃẄẅẞỲỳ –—‘’‚“”„•…‹›⁄⁰⁴⁵⁶⁷⁸⁹₀₁₂₃₄₅₆₇₈₉€™−◌]

Fixes #88

[m4rc1e]

@madig still interested in this?

[madig]

Sorry, I dropped off of GitHub recently.

Something's not going as planned when I paste the string above into a file:

❯ poetry run diffenator2 diff --fonts-before ../bla.ttf --fonts-after ../blaa.ttf --characters-file a
ninja: error: build.ninja:27: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:29: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:28: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:29: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:29: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:29: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:28: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:28: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:30: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:29: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:27: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:28: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:28: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
ninja: error: build.ninja:27: bad $-escape (literal $ must be written as $$)
      [!"#$%&'\(\)*+,-.\/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ'\[\...
          ^ near here
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/nikolaus/diffenator2/src/diffenator2/__main__.py", line 100, in main
    build_index_page(args.out)
  File "/home/nikolaus/diffenator2/src/diffenator2/html.py", line 178, in build_index_page
    assert len(html_files) > 0, f"No html docs found in {fp}."
AssertionError: No html docs found in out.