Adds .html.erb to erb language extensions for toggleTags command

[kjleitz]

Previously the ctrl+shift+` keybinding did not work out-of-the-box in *.html.erb files (the default extension for erb views in Rails). This PR adds that file extension to the erb language definition in package.json.

This may have been the source of #5

[seanharmer]

Any chance of getting this merged please?

[jonathansimmons]

@vortizhe This really is a necessity for this extension. Please merge.

[kirillplatonov]

Fixed in my fork: https://marketplace.visualstudio.com/items?itemName=kirillplatonov.erb-toggle