Add bower migration

[f-f]

Fix #159

This reworks the prototype from #272 to provide a smooth migration from Bower when doing spago init.

Excerpt of output in case some packages are wrong/not-found (the packages are totally fictional):

Spago encountered some errors while trying to migrate your Bower config.
A Spago config has been generated but it's recommended that you apply the suggestions here

It was not possible to parse the version range for these packages:
* "purescript-prelude" had range "^wrong"

These packages are missing from the package set. You should add them in your local package set:
(See here for how: https://github.com/spacchetti/spago#add-a-package-to-the-package-set)
* "arrays"
* "assert"
* "exceptions"
* "foreign"
* "foreign-object"
* "generics-rep"
* "globals"
* "nullable"
* "record"
* "typelevel-prelude"
* "variant"

TODO:

• Fix README
• Add tests