add 'forceCompile' rules modifier (#857)

Compilers that use data from sources other than local files may need
to be recompiled, but Hakyll's file-based dependency checking does
not handle this situation.

Add a new kind of dependency called 'AlwaysOutOfDate'.  If an item
has this dependency, it will be unconditionally rebuilt.

Also add the 'forceCompile' rule modifier, which is a user-friendly
way to force recompilation of specific items.  Example usage:

    forceCompile $ create ["foo"] $ do
        route $ idRoute
        compile $ unsafeCompiler $ doSomeIO

0 files changed, 0 insertions, 0 deletions