Feature request: keyboard shortcut for mark line

[Lladnaar]

Currently the only way to mark a line is to mouse click on the bullet to the left of the line. It would be useful to be able to do this by keyboard, especially when navigating using the vi commands. M would be easy to remember, and be similar to vi.

[variar]

m was added as shortcut for mark line in 7552c461e028a0f633c5eebbcb1a22a5ac240290

[Lladnaar]

Thank you.