Add support for external instrumentation file

[rasjani]

Currently, the instrumentation file is generated by release time and shipped within the python package.

It would be beneficial that users of SeleniumTestability to provide their own instrumentation.

One approach could be to publish the js part of our code as npm, then allow people to publish (or have a local copy) their own copy which could be taken into use at runtime to pointing some sort of environment variable to the package.