Implement focused and pending groups

[raniejade]

This resolves #19, resolves #21 and bring a lot of changes internally.
Example groups can now be tagged and its children will automatically inherit
those tags. before, after and around hooks now gets called for every
example group as well.

[codecov-io]

Current coverage is 88.56%

Merging #27 into master will increase coverage by +0.31% as of 607f2ff

@@            master     #27   diff @@
======================================
  Files           23      24     +1
  Stmts          383     411    +28
  Branches        63      73    +10
  Methods          0       0       
======================================
+ Hit            338     364    +26
- Partial         18      24     +6
+ Missed          27      23     -4

Review entire Coverage Diff as of 607f2ff

Powered by Codecov. Updated on successful CI builds.