Adds convenience methods to resolve UrlPatterns.

* UrlPatterns without parameters will implicitly convert to AppLocalUrl.
* UrlPatterns with parameters can be resolved using indexer notation.
  (Square brackets after UrlPattern expression to resolve.)
* Removes redundent extension methods now parameterless UrlPatterns
  implicitly convert to AppLocalUrl.
Replaces overloads with optional parameters.
Cleans up 'Urls' class: adds docs, removes 'Nullable' methods.

* Adds some doc-comments
* Adds Bool type class
* Removes Nullable method, since we already have the 'Or'/'OrNull'
  extensions methods which do the same thing less ambiguously and more
  tersely.
Changes how NullableComponent works. Does not allow null replacement strings.
Reworks 'example' project to be something bit more meaningful.

The 'Example' project is now a small, working (albeit completely trivial)
example. The examples before were really contrived, so hopefully this
makes it easier to see the actual advantages of using the library.