Print the current inserting line

This patch allow to store and reprint the current inserting line. This is done by appending a suffix character to the input. The line is (obviously) not sent.

In a nutshell:

:nick baz\
:nick bazqux
clamiax : 2015-10-09 18:15 >< NICK (): bazqux

This is useful, for example, when receiving data from the server in the middle of a sentence you're writing.

Note: the patch also changes the config.def.h file.