Update SCM commands to add/remove files.

[jacebrowning]

In Git, for example:

doorstop edit REQ --> git add PATH/TO/REQ999.yml
doorstop add REQ --> git add PATH/TO/REQ999.yml
doorstop remove REQ999 --> git rm PATH/TO/REQ999.yml

---

TODO:

• call edit where appropriate
• call add where appropriate
• call delete where appropriate (move item/document delete into the decorator)
• add tests for expected calls (split up class for those that do and do not require a VCS)
• remove coverage pragmas for ignored paths
• check for unnecessary test decorators

[jacebrowning]

This fixes #30.