add `0` key for beginning-of-line

This adds a shortcut for the `0` key to take the user to the beginning
of a given line, like Vim. It simply calls `line_home` like `^` does,
but this one is in my muscle memory from Vim usage.
Stop clobbering modifiers for digit keypresses

Previously, when `0` was used to call `line_home`, this would clobber
the modifiers even if a digit 1-9 was pressed before `0`, which should
signify the beginning of a modifier key sequence and cause `0` to behave
as a modifier until the next command is pressed.

This checks if a current modifier is in effect, and if so it does not
treat any digit keypresses as commands until the modifier is cleared.
Ignore illegal/unhandled keys
Merge branch 'nathantypanski-master'