Fix link

Thanks for the help at https://github.com/assemble/handlebars-helpers/pull/113#issuecomment-34836647 ...
Merge pull request #114 from dandv/patch-2

Fix link
withHash helper and tests
withHash helper documentation
Merge pull request #117 from mistakster/with-hash

withHash helper to create a new context from the attribute hash
Prototyping new docs format with YAML front-matter