Add middle click copy (#491)

Add middle click copy (#491)

* Add middle click copy

* Make middle click action configurable

This introduces the NNN_MIDDLECLICK environment variable, containing the
key that middle mouse click emulates. Only the first character is read.

* Document NNN_MIDDLECLICK in nnn.1

* Use NNN_MCLICK instead of NNN_MIDDLECLICK

* Add '^key' syntax to set middle click to Ctrl+Key