Add sublime-syntax for Nix expression language

docs(changelog): fix formatting in v0.7.0 changelog

This is based on [1] and converted to sublime-syntax using [2].

[1] https://github.com/wmertens/sublime-nix
[2] https://github.com/aziz/SublimeSyntaxConvertor

v0.7.0

This reverts commit 1815155c1d.

Closes #667

Add some missing words to the creating a theme page

strip wrapping whitespace from newline outward from shortcodes

Update t-rex.tileserver.ch

Sort sitemap elements by permalink

This feature was originally reported in #257 and got fixed in
3a2dab59743e8d193a80ddb1aa1932e1b5462322, however it seems this
got lost during development.

Closes #661
Ref #257

use svg icon in comparison table, tighten notes prose

specify proper sitemap schema

update zola version on deployment docs

use travis "minimal" vm in the documentation

If not specified, Travis use a system with ruby, which we don't need. We can use the "minimal" one instead.

Closes #530

Closes #636

v.0.6.0

Allow default base-path command line option to be set for building and serving.

Close #633