Added command line options to use XTERM palettes.
Merge @vrtsds's custom fixes(https://github.com/vrtsds/libsixel) into feature-builtin-palette

Conflicts:
	converters/img2sixel.c
Introduce -b/--builtin-palette option instead of -x/-y options
Merge branch 'develop' into feature-builtin-palette
Merge branch 'develop' into feature-builtin-palette

Conflicts:
	converters/img2sixel.c
Update optstring which is passed to getopt()
Amend fixes for -b option
Fix for passing manpage test
Update .travis.yml
Merge branch 'feature-builtin-palette' of github.com:saitoha/libsixel into feature-builtin-palette
Update .travis.yml
Fix test for MinGW environment
Rebuild
Merge branch 'feature-builtin-palette' into develop