Fixes #283 Find a way to map `<SPC> m` to `,`

Better version from danielwuz
Update achievements
use emmet as yasnippet source
Add Clojure and C-C++ to the "Batteries Included" list and turn them
all into hyperlinks
fix(last-buffer): move last buffer to <leader>bb

Fixes #301
Minor documentation change
Add basic helm-spacemacs extension on `<SPC> f e h`

Accessible via `<SPC> f e h`, it offers three actions:
- open README.md
- open packages.el
- open extensions.el
fix(tab): add neotree to spc tab
Move neotree toggle key binding definition to init-neotree
Define `<SPC> q` prefix command
Disable evil fine undo
Refactoring of config-system.el

Would be easier to improve helm-spacemacs and code
unit tests (in fact the pre-refactoring version was
not unit testable)
Support for packages in helm-spacemacs
Add Packages and Layer discovery section to README
Fix error with `git-enable-github-support` and helm-spacemacs