[FEATURE] Extend mustache template context using global options.context variable…

See merge request !4

[FEATURE] Extend mustache template context using global options.context variable or alter context of SVG data

Duplicated data URI encoding method

Whoops!  
The encoding method (`data:image/svg+xml`) is being added in the `urlEncode` function, so no need to repeat that on the object declaration.

See merge request !3

Improved URL encoding

I've borrowed [svg-url-loader](https://github.com/bhovhannes/svg-url-loader) encoding function to achieve an optimized URL encoding.  
The diffreneces can be seen in [this blog post](http://codepen.io/tigt/post/optimizing-svgs-in-data-uris).

See merge request !2

Readme fixes

I've fixed some typos, and corrected the usage example to be inside a gulp task.

See merge request !1