Add native mocha test execution support

[DinisCruz]

The core idea is that it should be possible to execute mocha tests directly from the UI. In fact one of the use cases for NWR should be to write and execute mocha requests.

• research other web-based mocha execution guis (which allow point a click execution of tests)
• see if the actually REPL script can be already a mocha test (i.e. the user thinks that it is writing just a bit of code, but behind the scenes that code is actually wrapped around a Mocha test which is then executed (this could be another 'context execution' option, just like the target language: Re-add javascript support (as a optional execution language) #13). Note that this is how the C# FluentSharp REPL code execution actually works, since behind the scenes what is happening is that the code written in REPL UI is wrapped around a full C# class)