Allow mouse selection override using ShiftMask

Similar to xterm or urxvt holding shift before selecting text with the mouse
allows to override copying text. For example in tmux with "mode-mouse on" or
vim (compiled with --with-x), mc, htop, etc.

forceselmod in config.h sets the modifier to use this mode, by default
ShiftMask.

Signed-off-by: Hiltjo Posthuma <hiltjo@codemadness.org>
added .gitignore file
initial commit of my config.h
added commented out Terminus font option
merged in changes to config.def.h
just track config.def.h
symlinked config.def.h to config.h
changed default font to DejaVu Sans Mono for Powerline
initial commit of my config.h
added commented out Terminus font option
merged in changes to config.def.h
just track config.def.h
symlinked config.def.h to config.h
merged in changes to config.def.h
just track config.def.h
symlinked config.def.h to config.h
initial commit of my config.h
added commented out Terminus font option
merged in changes to config.def.h
just track config.def.h