Add basic logic for opening program manpages

[RossSmyth]

This adds the comma -m arg, which allows comma to open manpages for you. Example usage:

❯ , -m rg

which then opens the ripgrep manpage.

I tested it on my machine and it seems to work fine.

[RossSmyth]

Maybe instead of a CLI flag it should be a subcommand, so it more closely replicates how man is used without comma, for example:

> , man rg

If for some reason a user wants to run , man, they can run , -- man and it will run the man program and not the subcommand.

[RossSmyth]

Let me know which is preferred.

[SuperSandro2000]

We already have -s for the shell, so -m is probably good enough for displaying the man page and I would not special case man subcommand.

[rszyma]

, man <cmd> is likely what people naturally try and it fails miserably (I did at least!). If we implemented this, it would be a nice UX, since it's so natural.