Added <mod-dependency /> to project.xml

[Just-Feeshy]

The <mod-dependency /> tag would allow users to include custom APIs in their Minecraft mod using PickHaxe.

Preview example of how the tag works:

<!-- The project.xml -->
<mod-dependency value="./dependencies/mcutils.jar" externLib="true" />
<mod-dependency value="./dependencies/seedutils.jar" />

# The generated build.hxml
--java-lib-extern
./dependencies/mcutils.jar
--java-lib
./dependencies/seedutils.jar