Added a handlebars helper to truncate a string

[elephantengine]

FUNCTION
truncate(str, length)

DESCRIPTION (WHAT)
Returns the first X characters in a string (unless it reaches the end
of the string first, in which case it will return fewer). Returns a
new string that is truncated to the given length.

USE CASE (WHY)
As a merchant, I want to display a card, at the bottom of
my home page that highlights the most recent blog post. In the card,
I want to display the blog thumbnail, title and the first 40 characters
of the post's body. In order to extract the first 40 characters, I need
a Handlebars helper that works like the javascript substring() function.

USAGE
{{lang (truncate 'blog.post.body.' 40) }}

@mcampa , @junedkazi , @mjschock

[mcampa]

Looks good to me. Do you mind writing a simple test case in /tests/helpers?
take a look at this example https://github.com/bigcommerce/paper/blob/master/test/helpers/concat.js

@bc-mkatz can you add this new helper to the docs?

[mQkatz]

@mcampa: Looks straightforward, will add this to the docs later today. Thanks.

[mcampa]

Awesome, thanks for writing the test. Just one more thing, can you fix the lint errors?

/home/travis/build/bigcommerce/paper/test/helpers/truncate.js
  16:7  error  Expected indentation of 8 space characters but found 6  indent
  17:7  error  Expected indentation of 8 space characters but found 6  indent

[mcampa]

👍