Plugin zip support

[BigFunger]

Fixes #4720

Adds zip support to the plugin installer.

[tsullivan]

I think for modules, you want to have those declared with const

[epixa]

Two things:

1. I cannot get the .zip extraction to work as expected, and I'm not sure if it's due to some mechanics of zip itself that I'm not familiar with or simply the way the contents are being extracted here. In any case, if I unzip the contents of a zip using OS X, I get a correct plugin directory structure. If I run the same file (test-plugin-master.zip) through this installer, the contents of the plugin get installed to installedPlugins/test-plugin/test-plugin-master/ rather than in installedPlugins/test-plugin. This happened for both a zip I created manually and the one you provided me. The installer runs fine, but warnings are thrown when starting kibana.
2. I really think this code needs some tests. At the most, all I can do is confirm that it works as-is, but without tests we have zero confidence that this continues to work as we expect going forward.

[w33ble]

You're not escaping the .'s, and I'm pretty sure that's a typo. Unless you really are trying to match filenamextarxgz

[simianhacker]

All these lets can be changed to const