Docs update

Stuff had got quite out of date. Why is it so much easier to write code
than docs?
Fix for the link macro to work around iPhone annoyance

This prevents Mobile Safari from sliding the address bar into view
whenever a tiddler link is clicked
Make the tag pills a little flatter
Comment clarification
Remove redefinition of Tiddler.hasField() and added Tiddler.isPlugin()

hasField() was already being defined in boot.js
Docs update
Ignore shadow tiddlers in the syncer
Update plugin loading mechanism to allow plugins to be unregistered

We still have to explicitly unpack them after registering or
unregistering
Allow themes to be switched by changing the tiddler `$:/theme`
Add an alternate theme

No UI for switching themes yet, try these two commands in the console:

`$tw.wiki.addTiddler({title:"$:/theme",
text:"$:/themes/tiddlywiki/starlight"})`

and

`$tw.wiki.addTiddler({title:"$:/theme",
text:"$:/themes/tiddlywiki/snowwhite"})`