Processing versions in configuration files

Several configuration files (.app.src, reltool.config) contain version tags that can be replaced by rebar with concrete values.

The version can be specified as follows:

raw	Result in processed file
"string"	"string" (no processing)
git	git describe --always --tags
p4	echo #head
hg	hg identify -i
bzr	bzr revno
svn	svnversion
fossil	fossil info
{cmd, Cmd}	result of executing Cmd as a script