Allow baseUrl for require.js to be configured dynamically

By not specifying a default and instead using the default behaviour
(when using data-main, as the entry point) as noted here:

http://requirejs.org/docs/api.html#config-baseUrl

Projects that extend the CWRC-Writer will be able to provide their own baseUrl
config.
Merge pull request #229 from nigelgbanks/issue-229

Can't configure baseUrl for require.js