Determine platforms to build based on buildPlatforms object.
Update build instructions to include how to build for specific platforms.
Add exclusive 'all' option to platforms list.
Switch to build:<target(s)> syntax.

Gave the build task some logic to map target platforms onto nodewebkit config parameters before running the task list.
Update build instructions with the new syntax.
Wording tweak.
Merge pull request #149 from OnAzureWings/specify_release_build_platforms

Allow user to specify build targets