Reuse existing Metron builds

[JonZeolla]

We shouldn't have to download and build fresh every time you run the script. Give the user the ability to use the latest Metron download, as long as it is a valid git repo. If it still needs built, build it, otherwise use the existing build.

[JonZeolla]

Future reminder: Don't forget that a prior or the current metron build may have used -p or -m. Perhaps this should be tracked in a flat file storing meta information?