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.
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