Added support for nerdtree's jump mappings + added a guard to prevent an error

[atrosekta]

the jump-mark way of getting back the selection after jumping may not be the the cleanest but it works for me, and the error that the guard fixed was probably known but unreferenced, you could reproduce it by entering a visual selection mode, including a "non valid node" line (like the " Press ? key for help or .. (up a dir) ) in your selection and trying to perform an action (like m, o, c, etc...)