Proposal: dub edit <dependency>: Automate the bugfix workflow

[FeepingCreature]

dub.selections.json has a feature where you can point a 
dependency at a filesystem path instead of a version.

Usually, when fixing a bug in a dub package, you have found the 
bug in the context of another project that uses that one. To test 
if your fix works, you can either edit the ~/.dub project, which 
is terrible, or redirect the dub selection path, which is 
annoying.

"dub edit" would aid with this workflow:

- check out the dependency at master in a subfolder of the 
current project (./.dub/wip/ maybe?)
- redirect the dub.selections.json entry for the dependency into 
that path
- possibly run some user-configured command (xterm? code .?) in 
the new path to open an IDE or shell.

This may seem a small change, but IMO common workflows should be 
trivial. WDYT?