More tab colouring tweaks
Refactor the HTML element parser

The purpose is to allow attributes to be specified as macro
invocations. For example `<div myattr=<<mymacro param1 param3>>>`. The
parser needed sprucing up in order to copy with the nesting of angle
brackets. The refactoring has been done with an eye on using the same
technique in the filter expression parser (which is pretty messy at the
moment -- it throws exceptions for syntax errors, which is bad). Later
I'm hoping to extend the technique to create a more flexible table
parser.