adjust if helper to support subexpressions

[mjschock]

adds the ability to use the if helper as a subexpression

context:
helpers/handlebars-helpers#224
https://github.com/assemble/handlebars-helpers/pull/225/files#diff-63430960261956d4a86cbd6cf21d229aR539

why:
to prevent this kind of thing: https://github.com/bigcommerce/stencil/blob/28eea2f9e8e88b4fdc43a2bc01c264d2989261f9/templates/components/products/card.html#L84-L98

instead it could be:

{{#or customer if(theme_settings.restrict_to_login '===' false)}}
            <div class="card-text" data-test-info-type="price">
                {{> components/products/price price=price customer=../../customer}}
            </div>
{{else}}
            <div class="card-text" data-test-info-type="price">
                <p translate>Log in for pricing</p>
            </div>
{{/or}}

or

{{#all theme_settings.restrict_to_login if(customer '!=' true)}}
            <div class="card-text" data-test-info-type="price">
                <p translate>Log in for pricing</p>
            </div>
{{else}}
            <div class="card-text" data-test-info-type="price">
                {{> components/products/price price=price customer=../../customer}}
            </div>
{{/all}}

[mcampa]

👍 LGTM