Compiled templates

[calvinmetcalf]

the L.Utils.template function currently takes a template and a string and returns the result, switching it to a compiled version based on @padolsey's example should increase the speed of the function.

For backwards compatibility L.Utils.template(url) could return a template function and L.Utils.template(url,data) returns templated string.

[tmcw]

Is template interpolation actually a bottleneck in any profiling results? This optimization is something of an improvement but is a ~40% boost to non-hot code worth the eval exception and complexity?

[mourner]

@tmcw I argued about this too (see PR comments), but eventually the pull could be simplified and cleaned up to a couple lines of code, so no big deal in regards to complexity. d0f768b