Add support for configurable HTML head and foot

This should implement the feature requested in gh-1.
Add example for the custom HTML head and foot
Document parameters for custom HTML head and foot
Mention that html_{head,foot} must be UTF-8-encoded character strings

Also add a test for this.
Merge branch 'feature/custom_head_and_foot'

This resolves gh-1 by implementing the requested feature of configurable HTML
head and foot.
Add Changes entry for new feature
v0.002000

 - Make HTML head and foot configurable