Make --self-contained able to fetch from https urls

[jgm]

As the title says.
Currently an error is raised.

[jgm]

Here are the options:

• automatically fall back to http (drawbacks: bad practice to make insecure requests when secure ones are requested, may not always be a fallback)
• use http-conduit (simple, but draws in a huge number of dependencies)
• use network-curl (simple, but requires libcurl C library, which may cause installation difficulties, esp. on Windows)
• just call the wget or curl command line program in shell

I don't love any of these, but it would be good to support https somehow. Is there an option I've missed?

[chloerei]

I hope odt and docx writer can fetch images from https urls too.

[jgm]

On reflection, I think the best solution is to add a cabal flag https; if set, http-conduit will be added as a dependency and used; otherwise, the standard http library will be used.