Add files from Bundler gem template

We're going to be releasing this project as a gem, so in preparation,
here we add the files generated by `bundle gem twigg`, with some tweaks
(replaced double quotes with singles, added some docs, removed the
dependency on Git, switching to autoloading of the version number etc).

It still is not suitable to ship as a gem (for example, we need to
rework the handling of the config file) but it is a necessary
precondition.

Change-Id: I41418e859b8dd1d4c19a29dcb2165c9983c31b43
Reviewed-on: https://gerrit.causes.com/24893
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>
Add md extension to README

This will cause GitHub to format it as Markdown.

Change-Id: I0dbf2195bce775b80c437d978797174550392f4f
Reviewed-on: https://gerrit.causes.com/24894
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>
Fix markdown syntax for number list

While we're at it, mark it up with blockquote syntax.

Change-Id: I24c5a5c40f2c3a6563aca3483d184570fdb29e81
Reviewed-on: https://gerrit.causes.com/24895
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>