Use % instead of // for directives.

Continuation of comment in https://github.com/codalab/codalab-cli/pull/29

>2) Worksheet directives: I don't think we should use '//' (I know I suggested it initially) for directives, because everywhere else in the CLI, it's been used for usage information for the user which are thrown away. '#' is no good either because that clashes with markdown syntax. There are two other considerations: (i) It'd be good to keep this consistent with the bundle metadata uploader, which uses '#' for usage information and ': '; (ii) it'd be good to have a natural way of handling line metadata. How about the following for both bundle metadata and worksheet editing?
•'//' denotes throw-away usage information
•'% : ' will denote metadata fields. For example: // Enter your information here...more instructions... % title: Overview of MCMC

The changes in this PR should support % for directives.

The current behavior for '//' is that all lines started with '//' are thrown away, meaning not persisted with the worksheet (https://github.com/cpoulain/codalab-cli/blob/directives/codalab/lib/worksheet_util.py#L156). I think this was the original behavior. Is that what you meant by throw away?