Add option to install from nimble file

[elcritch]

This PR adds an option to the install command to work from a local Nimble file. This makes it just a bit easier to install a package or dependencies in a CI or environment setup scripts. Doing a cd pkgdir && nimble ... can be annoying in some CI scripts. I figured an explicit option produced cleaner code and semantics than just checking if a pkgname ended with a ".nimble" extension. Though doing a local:// or file:// might another option.

Really it's not a necessary feature and partly I was motived partly by adding a small feature to Nimble to learn about it. If it's handy, great, if not no worries! :)

Examples:

./nimble install --depsOnly --nimble-file testpkg/testpkg.nimble 
./nimble install --nimble-file testpkg/testpkg.nimble 

[dom96]

Thank you for putting this together.

Two things:

• CI failing
• I think actually detecting whether the argument is an existing file would work quite well. Specifically restricted to .nimble files. No need for the --nimble-file flag.