better commit msg support

[extrawurst]

• allow moving cursor (see Add cursor to TextInputComponent for better commit message support (#46) #117)
• show chars used (>50 red)
• support delegating commit msg editing to a terminal editor for longer messages

[extrawurst]

Thanks @alistaircarscadden for implementing the cursor support:

[extrawurst]

see 477fdd3

[extrawurst]

@jonstodle thanks again, the external editor feature is released in 0.8 now: https://github.com/extrawurst/gitui/releases/tag/v0.8.0