Added new entity info_custommessage

Allows map editors to concatenate a sequence of strings and then
displays it using one of the pre-existing messaging entities, useful for
A = B/C messages.

Keys:
displayentity(targetname) : The entity used to display the message, can
be a targetname of a class game_text, point_worldhint or point_zsmessage
zsmessagemode(choices) : If using a point_zsmessage, select the method
of display with this key
"0"  : "Centred HUD Message"
"1"  : "Corner Human HUD Message"
"2"  : "Corner Zombie HUD Message"
string01(string) : Part 01 of the full string
string##(string) : Part ## of the full string, there can be up to 99 of
these.

Inputs:
displaystrings(string): Concatenates and displays the full string using
the attached entity
setstring##(string) : Sets part ## of the full string, there can be up
to 99 of these.

Outputs:
OnDisplayed : Called when the message has successfully displayed.
Added two entities to the FGD

+info_custommessage
+weapon_zs_butcherknife
Merge pull request #49 from BClark09/master

Added new entity info_custommessage and updated FGD