Add new one-column diff mode

2 columns is too much for small mobile screens so here's an alternative diff,
generated by new function, wikidiff2_inline_diff(). The PHP counterpart of it
is in Id7adcf34601b918a2c0a601a1a1690e602a5989b. This mode could be useful for
desktop too, so after a test run on mobile the functionality can be integrated into
core.

Additional test suite: https://github.com/MaxSem/DiffTest
...and results of its run: http://staging.wmflabs.org/diffs.html

Change-Id: I203eefd80dc4e22f55b7fc22b835dfa1212ec02d