add support for using existing Dash.app docsets

[smackesey]

Per my comment on #52:

My approach here was pretty simple, I added a new configuration variable DASHT_USE_DASH_APP that makes dasht use the Dash.app docsets. This also causes all of the docset management commands (install, update, remove, extract) to exit with a (new) exit code of 46. This seemed appropriate because if you are going to use the Dash.app library, it is best to just let Dash.app manage the docsets rather than trying to make sure dasht and Dash.app handle things exactly the same way.

One thing I wasn't sure of was whether to commit updated manpages. The Development section of the README says to rebuild the manpages, but the man directory is also in the gitignore. I can rebuild them and add them to this PR if you like.

[smackesey]

Hey, sorry this took me a little bit. I addressed all your requests except for the one about unnecessary array syntax. I'm not sure why exactly, but those particular globs won't expand in my bash v3.2.57 without the array syntax. Something about putting an * next to a quote, I think.

Also, did you want me to rebuild the manpages? See my question in my first PR comment.

[sunaku]

Thanks, I'll try to review this (free time is also scarce for me these days :sweat_smile:) but yes: please include the updated man pages.