support "preact"

[nippur72]

Preact is a React clone that is gaining popularity. With minimal changes, react-templates might be modified to support preact as a target different than React or React native.

From my early experiments the changes needed are:

• ability to use a different name than React.createElement. That is already implemented in PR #175, #179, #135, #93, #183, #184, #191, fix broken html tests, fix broken --react-import-path #176 with the new command line option --create-element-alias.
• pass props, state to the render function. This is one of the few differences between preact and react (and is optional too). The change should be easy since we already do something similar for stateless functions (which are function(props, context)).

As for how to enable the preact syntax, I am for using the already existing --target-version, by simply telling --target-version preact.

[motin]

+1 native preact support would be great.
@nippur72, have you checked if react-templates works with preact-compat?

[nippur72]

@motin I initially checked it when testing my PR, and if I remember correctly there were no problems. Of course the PR is to use preact without preact-compact.