Escape hash sing built-in function

[popinguy]

Creates an ability to generate scss file, which contains css classes such as
.{{name}}-background {
background: url('data:image/svg+xml;utf-8,{{#escapeHashSign}}{{{svg}}}{{/escapeHashSign}}') no-repeat;
}

Some svg-files contain hash signs ('#') which make it impossible to use such svg's as data urls. Url encoding of the whole file should work but It's not always suitable to increase generated file size. So I found it appropriate to encode only hash signs symbols. Using the library in cli mode makes it impossible to pass custom variables function to generator so it's the simplest way to deal with it.

[coveralls]

Coverage decreased (-0.2%) to 78.732% when pulling c7f5d97 on popinguy:master into 6966592 on jkphl:master.