Refactor vertical tabs for reusability

In practice, if you look at TabsMacro you’ll see that the width of the
tab content is wrong, and will usually overflow the container. Next
step is to fix that by using flexbox…
Use flexbox for vertical tabs

Thus stepping into a new world:

http://caniuse.com/flexbox

The vertical tabs are now completely reusable, with sensible wrapping
behaviour.
Add "before" and "after" filter operators

Fixes #357 by adding new “before” and “after” filter operators.
Remove obsolete filter operators docs

It was overwritten by a later version in editions/tw5.com/filters.
Update release note
Fix problem with variables containing an empty string

See
https://github.com/Jermolene/TiddlyWiki5/commit/e60fc9f81f2c8f0d115543d8
d330a1d68f9b890a#commitcomment-6301921
Clean up whitespace
Extend .tid files to allow single line text fields

To make it easier to create tiddlers that don’t include a terminating
newline in their text
Make permalink behaviour configurable

In the process introducing a new advanced settings tab

Fixes #600
Update reveal widget docs