Allow @jsx pragma to be set in .flowconfig

[Hubro]

I'm starting out a new project using flow and preact. The only way to make flow play nice with preact is to add // @jsx h to all my source files. It feels pretty silly to have to add the exact same flags to all my source files. It should be possible to set this globally in .flowconfig.

For example:

[options]
jsx.pragma=h

[goodmind]

Duplicate of #7010