* chore: Fix imlib2 Lua bindings
The API changed in a backward-incompatible way around 1.10 of imlib2
(though it's not clear which version broke it).
This resolves#1404.
Improved Lua API (lua_read_parse has been removed in favour of the
conky_parse() Lua API call). The docs have been improved to reflect the
recent Lua API stuff as well. Introduced new Lua Cairo and Imlib2
bindings, which require the tolua++ tool. Fixed some other
miscellaneous doc related things.