Tooltip placeholders supporting flot-axislabels plugin, flot plugin detection.

Added support for flot-axislabels plugin (https://github.com/markrcote/flot-axislabels) through two new placeholders "%lx" and "%ly" for respectively x and y axis labels.
If given without the plugin, the placeholders are removed from the tooltip content.
Added a plugin "detection" system for facilitating further plugin-dependent developments.