gruntfile: Allow a none platform to skip platform-specific tasks

[kiike]

A change in distPlatformTasks allows passing --platform=none so that no platform-specific tasks are executed. With this, we avoid running tasks that would not be necessary if, for instance, we are not going to use the zipfile or the AppImage, like the actions we carry out in the Nix package.

Without the --platform=none flag on linux64:

┌─────────┬─────────────────────────┐
│ (index) │ Values                  │
├─────────┼─────────────────────────┤
│ 0       │ 'jshint'                │
│ 1       │ 'clean:dist'            │
│ 2       │ 'nggettext_compile'     │
│ 3       │ 'copy:dist'             │
│ 4       │ 'json-minify'           │
│ 5       │ 'nwjs'                  │
│ 6       │ 'compress:linux64'      │
│ 7       │ 'shell:appImageLinux64' │
│ 8       │ 'clean:tmp'             │
└─────────┴─────────────────────────┘

With the --platform=none flag:

┌─────────┬─────────────────────┐
│ (index) │ Values              │
├─────────┼─────────────────────┤
│ 0       │ 'jshint'            │
│ 1       │ 'clean:dist'        │
│ 2       │ 'nggettext_compile' │
│ 3       │ 'copy:dist'         │
│ 4       │ 'json-minify'       │
│ 5       │ 'nwjs'              │
│ 6       │ 'clean:tmp'         │
└─────────┴─────────────────────┘

[cavearr]

One more! 👍