Use "atom-text-editor" tag instead of "editor[-colors]" classes

This fixes the following deprecation warnings:

- "Use the atom-text-editor tag instead of the editor-colors class."

- "Target the selector :host, atom-text-editor instead of
  .editor-colors for shadow DOM support."

- "Use the atom-text-editor tag instead of the editor class."

- "Target the selector :host, atom-text-editor instead of
  .editor for shadow DOM support."

Closes joaoafrmartins/seti-monokai#7,
Closes joaoafrmartins/seti-monokai#8,
Closes joaoafrmartins/seti-monokai#9,
Closes joaoafrmartins/seti-monokai#10.