Refactor Code & Stay Feature-Complete with garabik/grc

[exhuma]

This file contains a quick feature-analysis of garabik/grc and follows up with
a small prioritisation of implementation.

The following is a list of config-keywords (taken from the grc readme) and a
note whether it's used or not in the official config files.

Analysis/Review

Note: See the next section for prioritisation

• key: regexp (used everywhere: key feature)

  value: any regex

• key: colours (used everywhere: key feature)

  values:

  previous (used in only one instance: contrib/nmh-in-color/configs/conf.mail)
  Uses the same colour definition as the previous line. The documentation is
  unclear what "previous line" means. Is it the previous matched line? Or
  the previous line from the input?

  unchanged
  Used in many configs (example: "conf.id")
  "grc" expects a colour for each "group" in the regex match.
  "unchanged" is a placeholder for groups that don't need to be colourised.

  douple-quoted-string (Used only in conf.sql)
  Is injected literally at the beginning of the matched string. The docs say
  that it's eval'ed which I have not confirmed yet. It can f.ex. be used to
  support 256-colours (f.ex.: "\033[38;5;172m")

• key: command (unused)

  value: any console command

• key: concat (unused)

  value: a filename

• key: skip (used very rarely, example conf.ss)

  values:

  yes
  Don't include the line matching this regex in the output.

• key: replace

  values:

  any string with regex backrefs
  Replaces the current line with the template (using backrefs). All
  subsequent regexes will match against the replaced text

• key: count

  values:

  once (used sporadically, example=gcc)
  documentation is vague. It states:

  ... if the regexp is matched, its first occurrence is coloured and the
  program will continue with other regexp's.
  

  Does that mean that only the first occurrence in the line is colored? Or
  does it mean that it is only coloured once per stream? Not sure
  more (used in many places)
  When this is set, following regexes will be matched without advancing the line.
  stop (used sporadically)
  When this is set, immediately go to the next line on the input, ignoring any
  regexes following the "stop" rule.
  previous (only used once in a contrib rule)
  Reuses the same "count" rule as the previous line. The documentation is
  unclear what this means though: Previous matched line or previous line in
  the input (even if it didn't match).
  block (used sporadically)
  starts a block that all should have the same colour (until "unblock") is
  reached.
  unblock (used almost never)
  Ends a multiline-block that was started with "block"

Prioritisation

Following the above analysis, the features will be implemented in this order:

• regexp=... (key feature)
• colours=unchanged (key feature)
• colours=colour_1,colour_2,...,colour_n (key feature)
• count=more
• count=once
• count=stop
• count=block
• count=unblock
• Support config setting count=previous #3
• Support config setting colours=previous #2
• skip=yes
• replace=...
• Colour as double-quoted-string
• Verify if config setting command=... is really needed (currently unused) #4
• Verify if config setting concat=... is really needed (currently unused) #5
• Parse garabik-style config file into rules.
• Process stdin
• Process forked subprocess