Add first implementation of the spago docs command

[f-f]

This implements the first part of #89. It will generate documentation for the project files and its dependencies.

Example of running it:

$ spago docs --path "packages/*/src/**/*.purs" --path "apps/*/src/**/*.purs"
Generating documentation for the project. This might take a while..
Docs generated. Index is at "./generated-docs/index.html"