Proper escaping in mustache templates

The hastache templates used by `stack new` do not necessarily do adequate quoting / escaping when creating values in contexts like cabal or hpack files. See, for example https://github.com/commercialhaskell/stack/issues/3660#issuecomment-352204654 . In that case, the solution is to put quotes around it. However, this will not adequately handle backslashes.