Adds a new `-A`/`--show-all` option (in analogy to GNU Linux `cat`s option) that
highlights non-printable characters like space, tab or newline.
This works in two steps:
- **Preprocessing**: replace space by `•`, replace tab by `├──┤`, replace
newline by ``, etc.
- **Highlighting**: Use a newly written Sublime syntax to highlight
these special symbols.
Note: This feature is not technically a drop-in replacement for GNU `cat`s
`--show-all` but it has the same purpose.
This changes a few things:
- All syntaxes and themes are now stored (as submodules) under
assets/syntaxes and assets/themes
- The default directories for syntaxes and themes are "syntaxes"
and "themes" (used to be "syntax" and "themes")
- The "bat cache" command can now take a `--source <dir>` and
`--target <dir>` option.
- The cached files have been renamed to "themes.bin" and "syntaxes.bin"
