Change JSON instances for Opt to TemplateHaskell (#4083)

The `Generic` JSON instances for `Text.Pandoc.App.Opt` seem to tickle a
particulary bad quadratic complexity case (Generics complexity is worse
than quadratic with respect to the number of fields in the datatype).
This is with GHC-8.2.1, I didn't test it using 8.0 but I assume it is
similar.

Using `Generic`, compilation of the `Text.Pandoc.App` module takes
minutes and often gets killed due to out of memory on slower machines
with "only" 8GB of accessible memory.  This is particularly annoying to
me since it means I cannot build pandoc on Travis.

TemplateHaskell is a little uglier, but the module seems to compile
within a few seconds, and compilation doesn't take more than 1GB of
memory.

Should I also change the other JSON instances throughout the codebase
for consistency?

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