Indicating context change

[kallewesterling]

As @gofilipa's issue (DHRI-Curriculum/python#26) indicates, we need a way to signal context change in the lessons. Perhaps using a HTML comment:

<!-- context: terminal -->

<!-- context: text-editor -->

<!-- context: github -->

[kallewesterling]

This is now done in LESSON_TRANSPOSITIONS inside of the dhri.settings file.

An example:

LESSON_TRANSPOSITIONS = {
    '<!-- context: terminal -->': '<img src="terminal.png" />'
    '<!-- context: text-editor -->': '<img src="text-editor.png" />'
    '<!-- context: github -->': '<img src="github.png" />'
}