Make button invisible if plugin returns no output

[matthias-g]

This hides the indicator for a plugin if the plugin returns no output (see #49).

[aggsol]

This might be counter intuitive as no output would likely be an script error that should be visible.

[tallero]

I agree with @aggsol
@matthias-g Do you think would be better creating a setting to hide the plugin when there is no output?

[p-e-w]

Just a quick update: I haven't really made up my mind about this PR yet, but I won't be adding any more settings than there already are.

[matthias-g]

What do you think about considering also the exit code of the plugin and hiding the indicator only when the exit code is 0 and the output is empty? In case of an error the plugin should have an exit code != 0.

[aggsol]

@matthias-g What are you trying to achieve? It seems that you actually want to create something like a notification.

[matthias-g]

What are you trying to achieve?

One of my use cases is showing the battery level of my phone when it is connected. When the phone is not connected I don't want the plugin to take up space in the panel.

[matthias-g]

I didn't verify, but from looking at plugins like the Notmuch inbox & unread counter I assume that BitBar hides the button when there is no output or the first line of the output is "---". If that is true, adopting this behavior would be part of implementing BitBar compatibility.

[p-e-w]

@matthias-g Hiding the button when there is no output is problematic for the reason mentioned by @aggsol, but I like the idea of hiding it if the first line is a separator. I would merge a PR that implements that.

[matthias-g]

OK. I've added a commit that hides the button when the first line is a separator, but keeps showing the button when there is no output at all.

[p-e-w]

After addressing the above comments, please also squash everything into a single commit. Alternatively, I can do a squash when merging.

[matthias-g]

Thanks for the comments! I've performed the requested changes and squashed the commits.

[p-e-w]

Merged. Thank you for seeing this through, @matthias-g. This is definitely an improvement!