One may want to mount additional volumes to preserve certain directories
or to share some data between the host and the container. Allow users to
do so by providing a `-v` option that passes its arguments to
corresponding `-v`/`--volume` option for `docker run` command.

Part of #378.

Currently, the name of the Docker image is hardcoded in bst-here script.
This makes it harder for users to override it, which may be needed for
various reasons (custom plugins, private Docker registries etc.)

Add `-i` option to allow users to specify the base image. When `-i` is
not provided, default to `BST_HERE_IMAGE` environment variable if it's
set and otherwise to the current image -
`buildstream/buildstream-fedora`.

Also, re-order the command-line options in help text and source code in
alphabetical order to maintain sanity as the number of options is slowly
growing.

Part of #378.

No empty lines after a section title, 2 empty lines before a section title.

This is rather innacurate, you can build software in any programming languange
given that you have provided the required resources for this in the runtime
you are using.

As recommended by Valentin David in issue #332.

If we want to depend on being able to revision junction.refs and
project.refs separately, then we are better off just considering the
project.refs feature as available since the new version 8.

This will not harm projects which depended on it since version 5 instead.

Leave this error to be handled by preflight.

Updated test case to expect the new ElementError instead of a LoadError