Foldable xml

[samuelsinayoko]

Makes XML foldable a la display.JSON

See ipython/ipython#12415

[eseiver]

Hi @samuelsinayoko, thanks for the PR and apologies for the long delay in response! display_xml is something @mpacer and I threw together pretty quickly a while ago and I'm so glad it's been helpful to others.
Foldable xml is a great addition to the library, I have two questions/comments before this can be merged.

1. Thank you for creating the pytest component, but when I'm running locally both tests are failing.

Test 1 has AssertionError and Test 2 has TypeError: tostring() got an unexpected keyword argument 'pretty_print'

2. The expand parameter works as expected, but I was wondering about the displayed triangles for collapsing from your examples. When I ran it, I didn't have the triangle indicators. Is it because of an IPython plugin you're using?

I ask because for my particular use case, I wanted to display the xml fully expanded, but also without collapsed section markers. So in addition to the `expand` parameter, I might want something like a `expandable` param that would remove the folding quality entirely if desired.

Thank you again for your efforts here! ~Ellis