Adds config option `multimode`, which takes values `tab`, `vsplit`,
or `hsplit` (corresponding to the file-opening commands). I mean to
use it with a command line like
micro -multimode vsplit foo.h foo.c
to open files in a side-by-side split, but if one really wanted to
one could set it in the config file to change the default behavior of
opening multiple files in tabs.
Fix the 2nd part of #1773: resize via mouse drag doesn't work if the
split on the left contains other splits, i.e. is not a leaf node.
The problem is that only leaf nodes have unique id. For non-leaf nodes
ID() returns 0. So we shouldn't search the node by id.
So replace GetMouseSplitID() with GetMouseSplitNode().
