Add support for whitespace control character allowing for ERB like trim

[evulse]

Rebase of #743. To solve issues #215 and #216

[evulse]

Current performance from this feature

Warming up --------------------------------------
              parse:     2.000  i/100ms
        parse & run:     1.000  i/100ms
Calculating -------------------------------------
              parse:     27.043  (± 3.7%) i/s -      1.622k in  60.017511s
        parse & run:     17.996  (± 5.6%) i/s -      1.079k in  60.023723s

Master branch

Warming up --------------------------------------
              parse:     2.000  i/100ms
        parse & run:     1.000  i/100ms
Calculating -------------------------------------
              parse:     27.915  (± 3.6%) i/s -      1.674k in  60.023530s
        parse & run:     18.461  (± 5.4%) i/s -      1.107k in  60.020358s

[evulse]

@dylanahsmith @fw42 What is the best way to have an open pull request on the liquid-c repo so that this can use to fully build and pass tests?

[evulse]

This adds support for {{- and {%- syntax which will lstrip! and -}} and -%} which will rstrip!

[evulse]

Depends on Shopify/liquid-c#30 to pass all tests.

[fw42]

@Shopify/liquid, need some eyes on this (and Shopify/liquid-c#30)

[fw42]

would you mind rebasing and squashing your commits?