Handlebars support

[Glavin001]

Currently, the HTML beautifier will treat Handlebars as content. I believe js-beautifier does support indentation of Handlebars in HTML code. I will look into this further and get back with my findings.

Please +1 if you are interesting in this feature; I may even submit a Pull Request if it will help.

/cc @Frozenfire92

[Glavin001]

See https://github.com/einars/js-beautify/blob/219a44e96c92f7362143ff14925dd27c34640fb2/js/lib/beautify-html.js#L181

if (indent_handlebars) {
                        // Handlebars parsing is complicated.
                        // {{#foo}} and {{/foo}} are formatted tags.
                        // {{something}} should get treated as content, except:
                        // {{else}} specifically behaves like {{#if}} and {{/if}}
                        var peek3 = this.input.substr(this.pos, 3);
                        if (peek3 === '{{#' || peek3 === '{{/') {
                            // These are tags and not content.
                            break;
                        } else if (this.input.substr(this.pos, 2) === '{{') {
                            if (this.get_tag(true) === '{{else}}') {
                                break;
                            }
                        }
                    }

It looks like indent_handlebars is default false: https://github.com/einars/js-beautify/blob/219a44e96c92f7362143ff14925dd27c34640fb2/js/lib/beautify-html.js#L54

If this is configurable, then we are all set!

How about when the file extension is .hbs or .handlebars then it turns to true.

[Glavin001]

Reproduce this bug:

{{#if flag}} content1 {{else}} content2 {{/if}}

Then beautifier, it does not change. It should look like:

{{#if flag}} 
    content1 
{{else}} 
    content2 
{{/if}}

[Glavin001]

Moving to Glavin001/atom-beautify#13.