Refactor for 1.0

[madeleineostoja]

This issue covers everything needed for simpla-markdown to hit 1.0, both in terms of fixes/enhancements and upstream blockers.

Blockers:

• select filter on Simpla SDK gets (for Split raw markdown and rendered HTML data #16)
• Lightweight markdown editor component (for Replace SimpleMDE with custom editor #14)
• Simpla Anchor behavior (for Editor not positioned correctly if parent has transform applied #13 and Does not exit when clicked outside editor component (if parent transformed) #9)
• Simpla WC v1 / Polymer 2 support

Refactors:

• Replace SimpleMDE with lightweight editor ( Replace SimpleMDE with custom editor #14 )
• Replace HTML -> MD with split data ( Split raw markdown and rendered HTML data #16 )
• Position editor with JS rather than relative to parent ( Editor not positioned correctly if parent has transform applied #13, Does not exit when clicked outside editor component (if parent transformed) #9 )
• Polish editor UI ( Rework editor content styles #15 )
• Resources & tests ( Write docs and add demos #3, Add unit tests #4 )
• Port to pure Polymer 2 or Skate component

[madeleineostoja]

/cc @bedeoverend (re: blockers)