Add clp support

[jpe90]

This PR adds support for clp as an alternative to bat for file previewing.

In this implementation, clp is always used for file previewing if it's found. It assumes if the program is installed, the user would want to use it for file previewing, since that's its only function.

Alternatively a configuration option could be used to let users opt-in to clp more explicitly. Let me know if you would prefer doing it that way, or taking some other approach.

[junegunn]

Can clap show line numbers a la bat --style=numbers? I'd like to provide a consistent experience to the users regardless of the highlighter backend. We could use clp with nl, but then the numbers aren't highlighted.

[jpe90]

@junegunn not currently, but that sounds like a good idea to me. I'll work on it.

[jpe90]

@junegunn I noticed that bat line numbers don't seem to have background highlighting when they're on a highlighted line.

Just to clarify, were you referring to the grey coloring of the number line text?

[junegunn]

Just to clarify, were you referring to the grey coloring of the number line text?

Yes. With clp SOME_FILE | nl, we don't get highlighted line numbers.