Added support for onBuildRead.

[benquarmby]

Resolves #113.

[benquarmby]

This seems to be most logical place to put the onBuildRead hook, since it closes over all of the important variables starting with config.

[benquarmby]

I've rebased my changes on top of the latest master. I feel this is still a relevant pull request a year later.

[ramon18]

Indeed this pull request is still relevant 3 years later!
@jrburke Do you see any issues with this?

The current workaround is dynamically patch the "text" module:

 if (moduleName === "text") {
        var find = "text.finishLoad(name, parsed.strip, content, onLoad);";
        var replace = "if (config && config.isBuild && config.onBuildRead) { content = config.onBuildRead(name, url, content); }\n" + find;
        return contents.replace(find, replace);
      }

Thanks