Add a new overridable variable to the `run.sh` script to allow users to
specify extra volume bindings. If set, the variable should contain a
comma-separated list of bindings of the form:

  <directory on the host>:<mounting point in the container>[:options]`

Allow users to pass arbitrary options to the `docker run` command by
setting the `ODK_DOCKER_OPTIONS` variable prior to calling the `run.sh`
script.

Likewise, users of Singularity can set the `ODK_SINGULARITY_OPTIONS`
variable to pass arbitrary options to the `singularity exec` command.

Make the `run.sh` wrapper script check for a `run.sh.conf` file in the
`src/ontology` directory, and source that file if it exists.

This is to allow users to customize the behaviour of the wrapper script
by setting any of the ODK_* variable in that `run.sh.conf` file, instead
of having to export them in the environment or to manually set them at
each invocation of the wrapper script.

The `run.sh.conf` file is excluded from version control. It is intended
for local use only, not to be shared between all editors of the
ontology.