Add tags to tests

[luciano-renzi]

I want to add a list of tags to a test.

AC:

• A test can have a list of tags: tags = ['tag1', 'tag2']
• When running a suite tests can be filtered by tags
• A suite can have 'tags' attribute; add to GUI suite builder
• Tags can be passed to the run command: golem run foo bar --tags tag1 tag2
• -t argument should be used for tags instead of threads

[luciano-renzi]

Tags should work as a filter:

golem run project suite --tags foo  # run all the tests of 'suite' that have 'foo' tag
golem run project . --tags foo      # run every test with 'foo' tag
golem run project folder/ --tags    # run every test in 'folder' with 'foo' tag

[r-roos]

added via #167

[luciano-renzi]

0.8.0