Cleaning up the MoveUp and MoveDown functions

[hugomg]

This PR cleans up the code for moving things up and down, getting rid of some code duplication. Some of these ideas are from PR #27.

1. Replace pairs of MoveUp and MoveDown functions by a single MoveVertically function.
2. Don't treat the movement near the edge of the file as a special case -- a single call to min() or max() is all we need.
3. Get rid of the separate functions for moving things up and down by half a page.
4. Allow setting g:move_auto_indent to values other than 0 or 1

[matze]

Looks good, less code is always better 👍