Reorganize and add finishing touches for V3

[AlejandroFrias]

Changes:

• Update README
• achieve 100% coverage and add doctests to the test command
• Add lots of aliases (mixed, kebab, slug, spinal, screaming_snake)
• custom acronym detection now falls back to simple acronym detection when there are remaining strings of adjacent capital letters. e.g. snake('FOOBARBAZ', acronyms=['BAR']) now gives foo_bar_baz instead of f_o_o_bar_b_a_z)
• reorganized files structure to eliminate vague file names like utils and types
  • utils.py -> unicode_char.py, acronym.py
  • types.py -> the types were put closer to the code that owns the definition
• removed tracking of the original separator as it was never used, allowing for some simplification of variables and attributes stored and passed around