Add support for AliasMatch directives

(#479) Extend the _aliases.erb template so that the list of hashes in
`apache::vhost`'s `aliases` parameter can contain the usual `alias`
hashes but also `aliasmatch` hashes, which get translated to
`AliasMatch` directives.

Includes basic spec tests and updated documentation in README.md. Note
that these spec tests are currently only skeletons because rspec-puppet
drops constructs like '$1'. See the entries marked XXX.