Feature toggles

[tdammers]

Add Cabal flags so that some features can be enabled / disabled at compile time, such as:

• The shellout backend can pose security issues, so you may want to disable it
• The SQL backends introduce system library dependencies, which is inconvenient if you're not going to use them (e.g. depending, at runtime, on libmysql when you're never going to talk to a MySQL database)
• The HTTP backend could use HTTP implementations other than libcurl (it did in the past, but libcurl proved to be the most feature-complete one)
• There's probably a dozen more like these

Generally, this would be desirable to reduce dependency footprint, for security reasons, and to reduce overall binary size.

[tdammers]

• sqlite
• mysql
• postgres
• curl
• subprocess
• pandoc