Update shellcheck and shfmt to the latest version.
Fixes <https://github.com/koreader/koreader/issues/5152>.
Btw, you can apply shellcheck suggestions with a command like:
```
shellcheck --include=SC2250 -f diff *.sh | git apply
```
* Travis: speed up by caching base and running luacheck earlier
* ignore bin and install for git status change detection
* skip coverage except on official master branch. It adds 3 whole minutes and does nothing to prevent regressions
* also cache ~/.luarocks. It evens out but would generally prevent remote timeout shenenigans
* remove base cache dir before caching with verbose remove to see what's going on
* more inclusive shell code quality analysis
* fixed more shellcheck issues
* better shellcheck/shfmt debugging info
It turns out that one of our mupdf patch reads the FONTDIR env var
and uses it in a totally different way so we will use another env var
EXT_FONT_DIR to define external font directory for different platforms.
to support custom font directory for EPUB documents
Now Koreader could find fonts in the "fonts" directory in the USB root
directory of kindle, Kobo and PocketBook devices, thus no need to copy
fonts to "koreader/fonts" directory.
