Reorganized directory structure and changed Successor type.

Added URL parameter to Successor. This enables generating absolute URLs
from relative links.

Reorganized directory structure. Comics has been merged into Galleries.
In detail:

Comics.LinearComic => Galleries.Linear
Comics.List => Galleries.List

Added Galleries.Tree (empty).
Added Debug module.
Renamed Fetch.Iterating to Fetch.Tree
Added Galleries.Tree.
Renamed LinearComic to LinearCrawler.
Put the CLI into its own module.
Updated .gitignore.
Wrote Helper.String.combineURL.