New validation: mixed returnTypes

[p-bakker]

Produce a builder marker when a function returns mixed types

[angelozerr]

@p-bakker please create an issue per idea. It will more be easy to follow it. Thank's!

[p-bakker]

Done, also see #30, #31 and #32

[angelozerr]

Thank's @p-bakker for creating several issues. It should be cool if you can add a sample for each issue.

[p-bakker]

Sample:

function test(state) {
  if (state) {
     return true
  } else {
    return 'false'
  }
}  //Raise warning

[angelozerr]

@p-bakker as you have worked on JSDoc Tern support, do you think it's possible to study how to implement your lint issues.

I'm very busy with other tern topics, like custom completion. Thank's!