For each buffer with javascript file use all the javascript checkers from node_modules of the project

0. Find node-modules of the project. If not found, then do nothing: we're not in a project.
1. Read g:syntastic_javascript_checkers (For example, ['eslint', 'flow'])
2. For each checker find the executable in node_modules (If not found, show an error)
3. For each checker set b:syntastic_javascript_(checkername)_exec to the path found

Fugitive's diff doesn't work together: the plugin detects node-modules, but doesn't find the executables. If you know a way to fix this, you're welcome to PR!