Various tweaks and fixes

These should probably be separate pull requests, but oh well :grin:

* Kindle conversion profiles have been updated to include Kindle 4 and 5
* Default output dir for conversion is `images` for Kindle 5
* Code now works if `socks` module is not installed, and only fails if user specifies a proxy
* Bugfix: `--device` option works now
* MangaHere tweaks:
    * Fractional chapter numbers are supported
    * Manga search will wait 5 seconds if MangaHere asks for it
* Batoto support pulled in from another fork (requires BeautifulSoup 4)
    * Set up so that if the user does not have BS4 installed, Batoto is not provided as an option

I had also tweaked MangaHere to ignore the "next chapter" link in ongoing manga (as it doesn't exist), but your recent update negated it (and so I overwrote my tweak in my recent merge).