Add xml lexer performance improvement used in vis

[moesasji]

This adds performance improvements for the xml lexer added to vis in commits: 7e9e0a2ca868aaa214fb38a79fe71da34d6e00da and
baa51e934ce057af5b5be829d6a73a3e8b4c03d0

[orbitalquark]

This change does not correctly handle the following input: <size width=100 height=100> The "width=100" line will not be highlighted correctly, but the line below it will. This is why I had to use a function for look-behind. Note that currently there is no performance penalty because that look-behind function is not called. ('=' is not highlighted at all, and "=[number]" sequences are still highlighted, but not necessarily in the context of attributes.)