Registers additional Handlebars helper in Pattern Lab Node version.

Based on handlebars-helper-slugify

The slugify helper takes 1 argument: str

{{ slugify 'I am a title' }}

Renders to:

i-am-a-title

The bemModfier helper takes 2 arguments: block, modifier

{{ bemModifier 'block' [ 'modifier-1', 'modifier-2' ] }}

Renders to:

block block--modifier-1 block--modifier-2

The bemElementOf helper takes 2 arguments: block, element

{{ bemElementOf 'block' 'element' }}

Renders to:

block__element

The compare helper takes 3 arguments: value1 operator value2

{{#compare unicorns '!=' ponies}}
  I knew it, unicorns are NOT ponies!
{{/compare}}

The math helper takes 3 arguments: value1 operator value2

{{math 5 '+' 37}}

Renders to:

42