Command line args for juliainstaller

[davidanthoff]

juliainstaller should have command line args for

• choosing a default channel
• values for all configuration options
• a --force or --quiet or --yes option that installs with all defaults and no user interaction.

We should then also surface those flags in the shell script.

[davidanthoff]

See #253 (comment) for a roadmap.

[davidanthoff]

I now added --yes and --channel <CHANNELNAME>. We still need --force and then individual arguments for each of the configuration values.

The good news is that adding additional flags now is much easier, one just needs to add them here and then handle them in the juliainstaller code itself.