Added send-to options support to email-ext plugin

- added an ability to select the recipients from the predefined groups,
  such as 'Committers', 'Requester' and 'Culprits'
- added tests for the whole email-ext plugin

Change-Id: I3b03aa1f57f29352121b8501d1fa9fe77383f3b7
Merge "Added send-to options support to email-ext plugin"