[#46898331] First test - no source host defaults
[#46898331] Add desired unit tests for source_host

These reference the desired interface and behaviour.
[#46898331] Integration tests for source_host

These reference the desired interface and behaviour.
[#46898331] Logic of which source host to use

Unit tests pass, integration tests do not.
[#46898331] Add the source host to the message

This change passes the integration tests. However, once we do the next stage, i.e. having Tagalog calculate the source host, then this will become problematic as we cannot mock this in the test. One possibility is adding a --no-source-host flag, as per --no-stamp; another is refactoring the source_host method to do less. @dcarley and I will discuss tomorrow.
[#46898331] Simplifying to cover cmd line only

The reason we had to do this now is because it is blocking https://www.pivotaltracker.com/story/show/45081601. However, in order to move onto that we only need to make Tagalog take the source-host command line argument - we do not need to have Tagalog calculate the source host. That is actually not trivial, so I am separating that into a different story so we can continue with the higher priority https://www.pivotaltracker.com/story/show/45081601.
[#46898331] No source_host should be abst

Modifying behaviour based on code review - a missing source_host should just not appear, rather than being None.
[#46898331] Missed removing some Nones from tests
Merge pull request #4 from alphagov/46898331_tagalog_append_source_host

[Fixes #46898331] Support for @source_host