Node editor should be assigned via .gpirc

[nckz]

Currently the node editor for Linux environments is taken from the $EDITOR variable this should eventually change to the $VISUAL variable so that editors such as vim don't open in the gpi console. This could also be implemented as an option in the user's ~/.gpirc.

[nckz]

Could also check for xdg-open and prefer that if it exists. -then multiple windows can't be opened with the same file.

[borupdaniel]

Leaving this open as it seems like a nice feature to add to the to-do list.