Fix template + option processing semantics

- When creating a new entry redefining a default definition, options
  replace default options with the same key.
- The special option {'--', Keys} removes given option keys from the
  default specification.
- Added 'sys' message handling to exometer_proc, which means that
  `sys:get_status(exometer:info(Probe, ref))` will work.
Merge pull request #93 from Feuerlabs/uw-template-options

Fix template + option processing semantics