Remove options argument from link-to-asset

Was part of #3. Also controversial. But generally less so considering validation will stop people from misusing `precompile` and the normal use case does not require options being passed into `link-to-asset`.

**Update:** Not making options into precompile optional and removing options from `link-to-asset`. This requires a new middleware which wraps each request in a with-options. This is necessary because many ring servers use many threads and dynamic vars are thread local.