Fix RequireJS config reading

- Incorrectly named variable (context.options.mainRequireConfig -> context.options.requireConfig)
- Fixed regex on paths to account for slashes in the baseDir
Added ability to load multiple requireJS configurations from files and override with inline Gruntfile requireConfig, i.e.:

  templateOptions: {
    requireConfigFile: ['src/mainConfig.js', 'test/testConfigThatOverrides.js'],
    requireConfig: {
      param: 'this will override any param set in config files specified above'
    }
  }
Added requireJS 2.1.6
Updated README.md to reflect support for multiple requireJS config files
Merge pull request #30 from mhs/master

Better Support for requireConfig and requireConfigFile