Changed versiontag prefix to allow addition of dynamic data (i.e. date)

For example, you could set the versiontag prefix to be:

    git config --global gitflow.prefix.versiontag "production/\$(date +%Y/%m/%d/)"

which would result in tag names like:

    production/2010/09/22/my-hotfix-or-feature-name

NOTE:
Although this is a useful addition in the current version of git-flow,
in a future reimplementation of it in Python, this will be replaced by
hook scripts, so be sure not to rely on this feature too much for now.

git-flow-hotfix

@@ -39,7 +39,7 @@
 require_git_repo
 require_gitflow_initialized
 gitflow_load_settings
-VERSION_PREFIX=$(git config --get gitflow.prefix.versiontag)
+VERSION_PREFIX=$(eval "echo `git config --get gitflow.prefix.versiontag`")
 PREFIX=$(git config --get gitflow.prefix.hotfix)
 
 usage() {

git-flow-release

@@ -39,7 +39,7 @@
 require_git_repo
 require_gitflow_initialized
 gitflow_load_settings
-VERSION_PREFIX=$(git config --get gitflow.prefix.versiontag)
+VERSION_PREFIX=$(eval "echo `git config --get gitflow.prefix.versiontag`")
 PREFIX=$(git config --get gitflow.prefix.release)
 
 usage() {

git-flow-support

@@ -39,7 +39,7 @@
 require_git_repo
 require_gitflow_initialized
 gitflow_load_settings
-VERSION_PREFIX=$(git config --get gitflow.prefix.versiontag)
+VERSION_PREFIX=$(eval "echo `git config --get gitflow.prefix.versiontag`")
 PREFIX=$(git config --get gitflow.prefix.support)
 
 warn "note: The support subcommand is still very EXPERIMENTAL!"