Allow to disable network access via a cabal flag

[tarleb]

Checking the changes necessary to compile pandoc on wasm, it seems that the main changes are related to networking: haskell-wasm@336228b

I'd like to make it easier to compile pandoc to wasm, and to ultimately make wasm support part of the main branch. From what I understand, having a cabal flag network would be a step in that direction.

[jgm]

flags aren't supposed to affect the public API, and this might be impossible, given that PandocHttpError includes HttpException.

We could try to design around that somehow.
(e.g. change PandocHttpError so that it just uses a Text rather than a structured HttpException.)

[jgm]

Looks like this is going to be extremely difficult without some changes upstream: xml-conduit depends on network, and we depend on xml-conduit for XML parsing.