Add 'tags' filter api.

[tvandijck]

This is just an API that can contain a arbitrary set of values, which you can then filter on.
for example:

workspace 'foobar'
    configurations { 'release-std', 'debug-std', 'release-blz', 'debug-blz' }

   filter { 'configuration:*-std' }
        tags { 'use-std' }
   filter { 'configuration:*-blz' }
        tags { 'use-blz' }

   project 'test'
        filter { 'tags:use-blz' }
            includedependencies { 'blz' }
            defines { 'USE_BLZ' }

        filter { 'tags:use-std' }
            defines { 'USE_STD' }

It allows authors to expose features in libraries through tags, and users to enable/disable said features.

[starkos]

Huh, interesting. Yes, I'm good with this.