Add support for image trimming

[tpisani]

This adds a way to explicitly apply trim options. Another way to apply trimming is to prepend trim/ to the imagePath, but when combined with filters (since trim/ is now considered a part of imagePath) the final generated URL puts the filters before the trim which is not the way Thumbor expects the parameters to be organized causing an error response.

Example of an incorrectly formatted URL:

var Thumbor = require('thumbor')

const t = new Thumbor('KEY', 'http://thumbor.com')

t.setImagePath('trim/http://imgserver.com/image.png').filter('colorize(100,100,100,ffffff)').buildUrl()
// http://thumbor.com/-WBeY7C5iB0GJqFsu1HESfCi7rs=/filters:colorize(100,100,100,ffffff)/trim/http://imgserver.com/image.png

Examples of URLs using trim options:

var Thumbor = require('thumbor')

const t = new Thumbor('KEY', 'http://thumbor.com')

t.setImagePath('http://imgserver.com/image.png')

t.trim().buildUrl()
// http://thumbor.com/TaFyTswL4p1WRg9br4_rVv_hecY=/trim/http://imgserver.com/image.png

t.trim().filter('colorize(100,100,100,ffffff)').buildUrl()
// http://thumbor.com/9usuNQ8OniFoZ54aFf2j-Agmfl4=/trim/filters:colorize(100,100,100,ffffff)/http://imgserver.com/image.png

t.trim('bottom-right').buildUrl()
// http://thumbor.com/7EYHJRUQgH9LdB2e_BA-mzB5nsw=/trim:bottom-right/http://imgserver.com/image.png

t.trim('top-left', 30).buildUrl()
// http://thumbor.com/MMTAuxAU4E6TK79qVSHniOY9t_A=/trim:top-left:30/http://imgserver.com/image.png