Support for horizontal lines and a min/max range callback function

Added support for horizontal lines on charts (for y-axis, thresholds, etc), and custom range callback functions for greater control than the simple min/max value options.

Also fixed a 'bug' where values were clamped to the visible range, offering misinformation to users.

Can now specify partial grid options in SmoothieChart constructor.

`customisation.html` extended to demonstrate some of these additions.

Addressed a TODO in the code.

Obligatory whitespace commit!

More details in the commit messages.