Support External ERB templates on BES

[jvazquez-r7]

On #4726 we're playing with the idea of keeping JS/HTML files out of the ruby module for browser exploits.

Since BES supports ERB, it's kinda feasible! Would be nice to play with this idea and add further support on BES. In this way we could be able to write browser exploits as HTML/JS projects, write JS libraries for exploitation which should be easier to reuse, etc.

Additionally, would be nice to:

• merge browserautopwn and BES
• Allow JSObfu (unless already does it and I'm being dumb) to obfuscate several JS files and use them together (see Add module for EDB-35948, X360 Software actvx buffer overflow #4726 too).

[joevennix]

Tracked on the jsobfu repo: rapid7/jsobfu#8