Make it possible to pass `true` to sortable

Passing an argument of `true` to `sortable` on the table will use the
default sort function for all columns
Update README with docs for `sortable={true}`
Merge pull request #19 from glittershark/gh/13/sortable_true

Make it possible to pass `true` to `sortable` argument
Fixes #13
Merge branch 'master' of https://github.com/leesalminen/reactable

Conflicts:
	build/reactable.js
Update README.md

Updating readme.md to document native sort functions.
Update README.md

formatting is bad :(