Add reference links support

This commit resolves #184
fix links error
Improve code syntax for links assignment
Merge remote-tracking branch 'upstream/master' into ref_links
pass 'rawPath' page path to plugins
Add custom link capability

Users can add custom links at the top of the table of contents by
adding a links.custom object to book.json. Issue #304
update readme with custom links
Highlight active page in summary
Hooks for summary data
Support importing code snippets

Allowing to keep code in separate files, which can be cleaner in a
quite a few scenarios
Support code includes for exercises
Add tests for includes
Merge pull request #356 from GitbookIO/feature/includes

Support importing code snippets
Merge pull request #354 from citizenmatt/summary_hooks

Plugin hooks for summary data
Merge pull request #353 from citizenmatt/active_page

Highlight active page in summary
Merge pull request #347 from rlmv/custom_links

Add custom links to TOC
Merge pull request #268 from rlmv/master

Fix page.path for plugins
Merge pull request #266 from shaform/ref_links

Add reference links support
Add base command git:push and git:remote
Merge pull request #358 from GitbookIO/featureplatform

Add base command git:push and git:remote