Add flag to separately lock saved dependencies/devDependencies

[bribeney]

Behavior when locking with the -s or --saved flag is as follows:

• Projects originally saved as dependencies, and their dependencies,
  are locked under dependencies
• Projects saved as devDependencies, and their dependencies, that are
  not also in dependencies are locked under devDependencies
• Projects installed in bower_components that cannot be traced to
  dependencies saved in bower.json are omitted and a warning is printed
• If verbose flag is used, dependencies, devDependencies, and unsaved
  project names are listed

Behavior without the flag remains unchanged.

This is useful for workflows that use bower install --production or tooling that relies on the distinction between devDependencies and dependencies.

[bribeney]

I did run into one problem when testing this that involved an outdated dependency. Here's an illustration:

A depends on B and C v2
B depends on C v1
C v1 depends on D
C v2 does *not* depend on D

When for project A, bower installs B, C v2, and D.

D cannot be traced to any of the actually installed projects from saved dependencies, and bower list labels it "extraneous." Such extraneous dependencies show up as unsaved when using the new --saved flag, are excluded from locking, and their presence in bower_components without being listed in bower.json will cause bower-locker validate to fail. As far as I can tell, there's no local information bower-locker can use to identify and ignore this edge case.

bower-locker lock -s will still lock the file and simply issue a warning about the unsaved dependency, so the new feature still works in these cases.