Custom section ordering and support.

Users can now define their own sections and ordering. For example

    import_heading_stdlib = Standard Library
    import_heading_thirdparty = Third Party
    import_heading_firstparty = First Party
    import_heading_django = Django
    import_heading_pandas = Pandas
    known_django = django
    known_pandas = pandas,numpy
    known_first_party = p24,p24.imports._VERSION
    sections = FUTURE,STDLIB,DJANGO,THIRDPARTY,PANDAS,FIRSTPARTY,LOCALFOLDER

would create two new sections with the specified known modules.

    # Standard Library
    import os
    import p24.imports._argparse as argparse
    import p24.imports._subprocess as subprocess
    import sys

    # Django
    from django.conf import settings
    from django.db import models

    # Third Party
    from bottle import Bottle, redirect, response, run

    # Pandas
    import numpy as np
    import pandas as pd

    # First Party
    import p24.imports._VERSION as VERSION
    import p24.shared.media_wiki_syntax as syntax