Add FLOWLIB_ROOT variable to flowconfig

[goodmind]

This would allow ignoring specific libs

# node.flowconfig
[ignore]
<FLOWLIB_ROOT>/dom.js

# browser.flowconfig
[ignore]
<FLOWLIB_ROOT>/react.js
<FLOWLIB_ROOT>/react-dom.js

[goodmind]

Ignoring core.js should probably enable no_flowlib automatically. Or just ditch no_flowlib entirely and make it variation of

# no_flowlib = true
[ignore]
<FLOWLIB_ROOT>/.*
!<FLOWLIB_ROOT>/prelude.js

[include]
<FLOWLIB_ROOT>/prelude.js

# no_flowlib = false
[ignore]
<FLOWLIB_ROOT>/prelude.js

[include]
<FLOWLIB_ROOT>/.*
!<FLOWLIB_ROOT>/prelude.js

[goodmind]

/cc @gabelevi