Add option to overwrite the highlight_code filter

[danielfrg]

Since the highlight_code filter is created on the exporter from_notebook_node function it cannot be overwritten by the filters trait.

This makes it possible to overwrite it.

[MSeal]

Hi. Sorry no one was reviewing this for a bit. I'll try to contribute some more reviews now and then.
One thing to ask is if you could make a trivial test to prove filters get overwritten when we use this, it'd help keep the code quality up over time. Thanks!

[danielfrg]

Thanks for the review Mathew. I made the change you requested and added a small test.

[danielfrg]

Ok, I finally figured out why that particular test is failing. It's not about this PR it's about travis-ci installing the new ipython 7 version. Old builds used ipython 6.X.

In ipython 7 the traceback when you keyboard interrupt is:

...
<ipython-input-2-85e2e9e78df7> in <module>
...

While in ipython 6 it was (note the parenthesis):

...
<ipython-input-2-85e2e9e78df7> in <module>()
...

I can update the test notebook to be compatible with ipython 7 and pin the ipython used in testing to be >=7.

[danielfrg]

Ok, I decided to not pin the ipython version because it will break the tests for Python 2. Unfortunately then it required to duplicate some test files to test in IPython 6 and 7.

I ran the tests on master locally and they are failing because of this same reason so i bet if they get executed right now on travis it will fail but this will fix it.

[MSeal]

Awesome. Thanks for looking into the test failure as well!