Add possibility to upgrade submodules

[presidenten]

A missing feature form npm is the possibility to update a npm modules submodules even if the npm module it self have not got a new version.

Consider the following scenario:
Project depends on a: ~1.0.0 that depends on b: ~1.0.0.
Thenb releases version 1.0.1 that contains a bugfix. To get it before a updates with npm, you can use npm install a --force.

This will refetch all the latest submodules that a uses. We need a similar command in yarn that would update all submodules and update the yarn.lock as well, even if the module it self has not changed.

(Please note that it is different from npm install --force, because it only acts on one module, instead of all modules.)

Using:
Yarn 0.19.1
Node 6.9.1
Osx sierra 10.12.2

[bestander]

Discussion is here yarnpkg/rfcs#54