Add Powerline support to the tracking package.

[travisbhartwell]

When the Powerline package is used, instead of modifying the
mode-line-format', the mode-line needs to be updated in global-mode-string'.

Updated the documentation for tracking-position to indicate this
behavior.

Fixes #137.

[jorgenschaefer]

Thank you for this contribution, very nice!

Please do not use (or <condition> <alternative>), that's highly unusual and reduces readability. I prefer (when (not <condition>) <alternative>). If you have a strong aversion against using not there, you can use (unless <condition> <alternative>), though I find unless generally pretty unreadable.

Also, is (featurep 'powerline) really the best way to check if powerline is enabled? It would return false positives when the powerline package is loaded but not used. A check like (memq 'mode-line-modes mode-line-format) might be more generic?

[travisbhartwell]

Thanks for the feedback.

I admit I was just copying the idiom I saw in timeclock.el and time.el, but I agree, using when is more readable.

I've force pushed a fix to my branch for that.

I'm still trying to determine the most reliable way to determine that powerline is active. I'll push that change when I figure it out and comment here so you can review that.

Thanks!

[travisbhartwell]

I couldn't come up with a reliable, general way to detect if Powerline is active beyond the use of featurep, so I filed an issue, milkypostman/powerline#97 to get guidance on how to do this or how to make a change to Powerline to support this.

[jorgenschaefer]

Thanks for that! In the meantime, do you think adding a setting like powerline for tracking-position would be a good idea, and then adjusting the default value of tracking-position depending on that?

Also, (memq 'mode-line-modes mode-line-format) as a check might be sufficient already …

[bradens]

Any progress on this one? I'd like to get using circe with powerline.

[travisbhartwell]

@bradens Sorry, I've been ill and haven't had the energy to get back to this, I hope to soon (next day or so). I do welcome help to make the suggestions @jorgenschaefer mentioned.

[theanalyst]

I would also like to see this feature in, @travisbhartwell will you be able to look into this?