fix get_undeclared_template_variables() to include header/footer

Fix elapouya#271

{%- xxx -%} merging feature

v0.8.0

Merge branch 'Dutchy--master'

Merge pull request elapouya#263 from Dutchy-/master

Make headers and footers work with both python-docx 0.8.7 and 0.8.10

Merge pull request elapouya#260 from komawar/fix-documentation-grammar

Fix grammar in index.rst

Merge pull request elapouya#258 from awesomo4000/fix-lxml-parse

Prevent lxml from attempting to parse None

Merge pull request elapouya#207 from edufresne/master

Truncate zip file before re-writing it for  replace_media with file-like objects

Merge pull request elapouya#205 from toloco/fix-spacing

Handle spaces correctly when run are split by Jinja code