Convert to ES6 compatible library

[AStoker]

ES6 Conversion + Features

Resolves #15 Added a new file (toastify-es.js) which contains an ES6 compliant version of Toastify.
Few improvements and changes to behavior:

• Based off comments, positionLeft was finally removed
• Expanded functionality for selector that allows you to pass in an element as well (defaults to document.body)

Notable changes to code:

• Made use of arrow functions to keep context the same (replaced (function(){...}).bind(this))
• let instead of var
• Toastify class rather than function. Exported function creates an instance of Toastify
• When checking classes, use the built in classList functionality on elements
• rootElement is now stored on the class so when we remove toasts we know where to look specifically (this enhanced the ability of putting toasts inside elements and allows you to use Toastify inside shadowDom nodes)
• Added ability to use escapeMarkup to allow optionally unsafe innerHTML
• Added ability to use the HTML DOM style property to directly style the toast (this further extends backgroundColor functionality, and therefore backgroundColor should be deprecated in the future in favor of using style.background). Note, since previous functionality took backgroundColor and applied it to the elements background style (which changes all background styles), to prevent breaking changes, backgroundColor takes precedence over style.backgroundColor.

Things that should still happen:

• [x] Get maintainer feedback (I'm only human and spent a few hours quickly changing things, don't want to miss something)
• [x] Prevent XSS issues when we pass text into the innerHTML of the toast (probably just want to use innerText, as there's already node for when you want to pass in an element)
• [x] Cleanup of default values and states
• [x] Adequate testing of all use cases
• [x] Test across browsers
• [x] Include added functionality in legacy versions

[AStoker]

@apvarun I think this is ready for review

[AStoker]

@apvarun, see anything I missed in this PR? Would love to get this first draft in. Next steps can be to create a Typescript version which can compile out to native modules or legacy javascript

[AStoker]

@apvarun , let me know if there's something I can do to help this get merged in faster

[apvarun]

I think we should also reference this new es file somehow so that the correct file is picked up by bundler