Add column selection information

The information in the unnoficial docs wasn't enough, and the one from the official docs is wrong.
Add column selection shortcuts
Add column selection shortcut to osx
Update source/basic_concepts.rst

Minor change: contens --> contents
Merge pull request #19 from colgur/patch-1

Minor typo in source/basic_concepts.rst
clarification on file naming
Merge pull request #22 from tobeportable/patch-2

clarification on file naming
Added some Mac keyboard shortcuts
Merge pull request #26 from bradobro/mac_keys

Added some Mac keyboard shortcuts
add save and prompt_save_as
added 'panel_has_focus' and 'panel' context keys
Merge pull request #28 from hectorh30/patch-1

added 'panel_has_focus' and 'panel' context keys
Merge pull request #27 from pscheit/patch-1

add save and prompt_save_as to sublime commands
minor style changes
Merge pull request #10 from PaBLoX-CL/master

Add column selection documentation
Document how to use `]]>` in snippets
Merge pull request #29 from dslatten/patch-1

Document how to use `]]>` in snippets
Update commands.rst

Adding "title_case" and "swap_case" commands
Merge pull request #31 from melanef/patch-1

Update commands.rst
edit