Add operators to perform calculations using the math/hobby module.
Allow names to begin like existing non-splitting tokens.

e.g: `Skew` is an operator and `SkewTransform` is an allowed name.
Parse 'Infinity' as a number
Allow negative indexes in :i pseudo class selector.
Merge pull request #247 from graphicore/improvements-for-formulae-language-and-selectors

Improvements to formulae language and selectors