Add support for insert key

Add support for insert key

Insert key stands for a key which allows enter or leaves insert mode, so let
it generates the correct sequence to change between these modes:

   - Insert: Enter in insert mode.
   - Shift + Insert: Leave insert mode (replace mode).
   - Control + Insert: Insert a blank line.

Like Shift + Insert also paste text, if a user want this feature be full
functional he has to modify such shortcut.
---
 config.def.h |   16 ++++++++++++----
 st.info      |    3 +++
 2 files changed, 15 insertions(+), 4 deletions(-)