Generalise internal function in chromosome diagram code
Refactoring chromosome diagram label code
Support optional fill color on chromosome diagrams
Test optional fill color on chromosome diagrams
Use feature fill colour as label background on chromsome diagrams

Based on an earlier hack from Oct 2011, which like the recently
committed fill color, was used in figures in Jupe et al. (2012)
http://dx.doi.org/10.1186/1471-2164-13-75
Test strand specific chromosome diagrams labels with background color