feat: Added dynamic partial blocks by adding support for empty block closing tags

[henriquecolini]

This PR implements a feature that has been historically lacking from the original Handlebars: dynamic partial blocks. handlebars-lang/handlebars-parser#9

Handlebars syntax allows users to create both dynamic partials:

{{> (snake_case 'MyPartial') }}

And partial blocks:

{{#> my_partial }}
    Hello
{{/my_partial}}

In theory, one should be able to use both features at the same time since there's no conflict whatsoever betwen them. In practice though, this turns out to be impossible due to a funny oversight in the syntax:

{{#> (snake_case 'MyPartial') }}
    Hello
{{/ ??? }}
what goes here?

I have fixed this issue by making the explicit name in the closing tag optional, which I consider the most obvious change to the syntax. So, this is now possible:

{{#> (snake_case 'MyPartial') }}
    Hello, this works now!
{{/}}

And as a bonus, it's also possible in general, for any partial and decorator block:

{{#> my_partial }}
    {{#*inline foo }}
        some inline stuff
    {{/}}
{{/}}

[coveralls]

coverage: 82.399%. first build
when pulling 1c92f2d on henriquecolini:master
into 888ed6a on sunng87:master.

[sunng87]

This looks mostly good to me. Just add two small comments

[sunng87]

@henriquecolini I just reverted the version change by myself. Thank you for the patch!