Simple API for in-memory templates

[takluyver]

Closes #673.

The new API is a method exporter.use_string_template(s), which you pass the string template into.

Example to use it with a subclass:

import sys
from nbconvert.exporters import HTMLExporter

POPPING_TEMPLATE = """
{%- extends 'full.tpl' -%}

{%- block markdowncell -%}
<div style="background-color: hsl(85, 86%, 76%);">
{{ super() }}
</div>
{%- endblock markdowncell -%}
"""

class PopExporter(HTMLExporter):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.use_string_template(POPPING_TEMPLATE)

if __name__ == '__main__':
    out, res = PopExporter().from_filename(sys.argv[1])
    with open('pops.html', 'w') as f:
        f.write(out)

[mpacer]

I had a slightly different API in mind, and I'm going to try to write it from scratch but if I find myself using enough patterns similar to yours I'll just rebase it off of this branch.