Added UrlHelperExtensions.
Added UrlHelperExtensions unit tests.
Changed name of helper method to GetLink, changed names of unit tests to match.
Added typeparam comment to method that was missing it per the
Added guard clauses to all 4 methods.
Added unit tests for the null checks in each GetLink method
Merge branch '21-add-urlhelper-extensions-redux2' of https://github.com/dlongest/Hyprlinkr into dlongest-21-add-urlhelper-extensions-redux2
Incremented minor version,

in order to indicate that the addition of an extension method on UrlHelper
is a new feature. This resolves #21
(https://github.com/ploeh/Hyprlinkr/issues/21).