Add settings option to enable force pushing to Github

[matthewjackowski]

Some workflows might want the flexibility to force push translations. Since git commits can happen outside of Txgh (an important component of Txgh is supporting parallel workflows)...Txgh commits could become blocked by conflicts. Force pushing 'solves' this only in the case where someone has 'designed' this to work and understands the implications.
Therefore by default force push should always be = false.

With great power comes great responsibility. - Spiderman