Make terminal colors optional

Previously, the script was hard-coded to print unix-style ANSI
sequences to colorize the terminal output. These don't work on standard
Windows consoles, though.

Add an option which lets you disable colorized output (there's
currently no alternative indication of checksum match/mismatch though).
Improve check/add output format

Improve the output format of the check and add subcommands, which
improves clarity both with coloring and without.

Output format for check subcommand is now:

    >xxx <CRC>  <SHA1>  <full path and filename>
     ^^^- SHA1 state glyph
     |\-- CRC (DB) state glyph
     \--- CRC (filename) state glyph

Output format for add subcommand is now:

    >x <CRC>  <SHA1>  <full path and filename>
     ^- CRC (filename) state glyph

The state glyphs are currently hard-coded, but will in the future be
configurable.

The state glyphs and their meanings are:
 * ' ' (space): state unknown (e.g. nothing to compare against because
               no checksum in filename or no db entry
 * '-' (dash):  database not available
 * '✓' (unicode tick mark): checksum matches
 * '⨯' (unicode x mark): checksum does not match
Merge branch 'feature/make-colors-optional' into develop