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
```
This commit standardizes the various todos around the code a bit in a manner recognized by LDoc.
Besides drawing more attention by being displayed in the developer docs, they're also extractable with LDoc on the command line:
```sh
ldoc --tags todo,fixme *.lua
```
However, whether that particular usage offers any advantage over other search tools is questionable at best.
* and some random beautification
Follow-up to https://github.com/koreader/koreader/pull/4524
The regex in the Python wasn't actually picking up on that style of writing it at all, because it's not only ugly, but also so counter-intuitive that I overlooked to test for and add support for it.
```
_(
[[
```
It'd be easy to fix up the Python regex a little, and perhaps I will,
but either way it makes more sense to automatically enforce this as a coding standard.
* shellcheck 0.4.5 fix `LC_ALL: en_US.UTF8` (can be removed for shellcheck 0.4.6)
* hush updated luacheck on `reader.lua`; we're not assigning any variables but `= nil` is redundant
