Add command bundle why, similar to yarn why

[wscourge]

It'd be nice to simply run bundle why gem_name and learn why the dependency is used - especially paired with bundle outdated.

Yarn does that with the yarn why pkg_name already. Here's an example output:

$ yarn why @ampproject/remapping

yarn why v1.22.19
[1/4] Why do we have the module "@ampproject/remapping"...?
[2/4] Initialising dependency graph...
[3/4] Finding dependency...
[4/4] Calculating file sizes...
=> Found "@ampproject/remapping@2.2.0"
info Reasons this module exists
   - "@babel#core" depends on it
   - Hoisted from "@babel#core#@ampproject#remapping"
info Disk size without dependencies: "92KB"
info Disk size with unique dependencies: "372KB"
info Disk size with transitive dependencies: "572KB"
info Number of shared dependencies: 4
Done in 0.95s.

Cheers, and keep up the good work.