editorconfig: don't globally force spaces for indent style.

Ran into some problems with existing projects that used tabs. Enabling
this setting forced a tabs-to-spaces conversion on save, resulting in
large-scale changes to whitespace in the existing code.
In future, define this at the project level.
editorconfig: prevent stripping of trailing whitespace in `.md` files.
atom: export path to `brew cask`-installed Atom.app

Atom apparently expects Atom.app in /Applications, so exporting
ATOM_PATH fixes the CLI tools not being able to find it.
sublime text: update installed packages.

Add: Emmet, SublimeLinter-jscs.
Remove: AutoWrap.
sublime text: move sublime-specific ignores to the top-level .gitignore.
sublime text: add SublimeLinter settings file.
sublime text: add user keymap file.
sublime text: :lipstick: fix whitespace in settings file.