1. combined loadTemplate + renderTemplate

2. stream through all files (sprite.svg, sprite.css, [optional] sprite.html), instead of some coming from stream, some being written by 'fs'
3. added `svgDest` option to allow configuration of the output .svg file name + path
4. changed to use some of the best practices of writting gulp plugin
5. fixed various bugs (e.g. end the stream)

Author: blai