Update kobo.py to include subtitle if field exists in custom columns

[dotknott]

Since Subtitle is becoming more common in Calibre DB setups with Kobo users, I added subtitle to the sync data when there's a custom column labeled 'subtitle'. When no matching column exists it returns an empty string.

[szkafen]

In my particular case, I wanted the subtitle field to show the number of pages as, e.g., "256 pages". This solution seems to only like columns with plain text or numbers. I made the #subtitle column be populated with integers from Count Words and it worked, however it only outputed "256", which was only half my desired end goal.

I tried making a new #subtitle column, having it referencing the number column and with the template {#pages:} pages. The output of the column was indeed "256 pages", but sync would fail allways. It seems it doesn't like templates.

However, I found a solution. Going back to making the #subtitle column be the one populated by Count Pages and changing line 440 to return str(subtitleColumn[0].value) + " pages" worked flawlessly, and outputed the desired outcome in the Kobo. Here, " pages" can be changed to the desired word (in my case, since I want to respect Spanish grammar, " páginas"). It could also be changed to return "Pgs: " + str(subtitleColumn[0].value) or whatever to show variants of this output.

[AsherMaximum]

I've made #3376 to make the subtitle sent to Kobo devices a config option in the Admin Feature settings, along with an option to add a suffix and/or prefix to it, which should cover both @dotknott's and @szkafen's use cases.