Add pre-fetch function to ajax module

[Antonio-Maranhao]

Overview

Allows setting a 'pre-fetch' function that is executed before every network request
made by the app/core/ajax.js module. It can be used to run actions before
fetch() is invoked by Fauxton, even allowing one to modify
the original fetch() parameters.

Testing recommendations

Unit tests provided to validate the new function.
Application behavior hasn't changed as the default pre-fetch function is a no-op.

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;
• Update rebar.config.script with the correct tag once a new Fauxton release is made