Fix problem with parsing main UI boot tiddlers

We were parsing the boot tiddlers, making them into a widget and then
refreshing the widget tree. The problem is that subsequent chances to
the boot tiddlers themselves wouldn’t be picked up as part of the
refresh.

Now we indirectly parse those UI boot tiddlers through a transclusion,
which does get refreshed in the desired way.
Update release note
Fix problem with list fields containing `[[]]`

Fixes #603 - thanks @xcazin!

It no longer crashes but unfortunately if you round trip a tiddler out
of edit mode and back you’ll lose any empty double square brackets.
Fix problem with parsing lists contain non-breaking spaces

Some of the time we need to treat non-breaking spaces as though they
are not spaces (regexps treat them as spaces by default).
Fix typos in docs for image parser
Implement explicit external links
Merge pull request #602 from BramChen/master

Add chinese translation of banner for binary tiddlers in edit mode
Upgrade codemirror to current master
Prioritize "module.exports" over "exports" in require sandbox
  (Node-ism, inherited (temporarily?) to support codemirror upgrade)
Fix up codemirror instructions to match new paths in config example
Fixes for running TiddlyWiki on node-webkit

See Jermolene/TiddlyWiki5NodeWebkit
Improve TiddlyWiki as a library

1. Make it possible to disable specific boot tasks
2. Extend the startup mechanism to allow startup tasks to be disabled

Again, see Jermolene/TiddlyWiki5NodeWebkit to see how these features
fit together.
Fix incorrect filter in shadow $:/DefaultTiddlers

Fixes #606
Merge pull request #604 from natecain/module_exports

Module exports
Pass content-type to codemirror mode option for syntax highlight

Fixes #605
Restored drag and drop functionality within TEXTAREAs and INPUTs

As mentioned by @Skeeve in #592
Remove erroneous global wiki references

$tw.wiki is used as a global reference in the outer initialisation
modules. It shouldn’t be used in widgets etc.
Fixed problem with widgets variable access

Previously, widgets were reading variables from themselves or their
cascaded ancestors. That means that if a widget sets a variable and
then reads the same variable, it will get the same variable back. That
sounds reasonable, until you consider a widget that wants to modify a
variable - eg the tiddler macro. For example:

```
<$tiddler tiddler={{!!report}}>
<$transclude mode="block" />
</$tiddler>
```

Here we first evaluate the `{{!!report}}` reference, which involves
reading the currentTiddler variable, looking up the tiddler, and
retrieving it’s `report` field. The next the tiddler widget is
refreshed, it will use the newly set currentTiddler as the basis for
resolving the `{{!!reference}}` reference.

The fix is to get variables from ancestors, but continue to set them on
ourselves.
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.