Add grunt options --startSymbol and --endSymbol to allow custom symbols.

See also #299.

One caveat here: the grunt task modifies the files in place. This means that everything in src/templates changes when `grunt build` is ran with either `--startSymbol` or `--endSymbol`. Problems I see with this:

- Someone might accidentally commit these changes to the templates.
- `grunt test` fails miserably without resetting the templates to the original symbols.

One workaround here would be to use something like [grunt-contrib-copy](https://github.com/gruntjs/grunt-contrib-copy) to copy the template files to a temporary directory, do the build using that as the source, then remove them in the clean task. I'm happy to add this if needed. Thoughts?