Permit variable template names in extends and include tags

[nimasmi]

Description

This permits the use of an extends tag

{% extends foo %}

or an include tag

{% include foo %}

where the context defines foo.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[bcdickinson]

Thanks for this, Nick. I've rolled it into #124 so I'll close this PR now.