[Danger] Add some static check rules in dangerfile

[wrmswindmill]

Brief Description of the PR

This PR add the following features:

1. Check if PR is submitted to master branch,if not ,the Travis CI will build fail.
2. Check if PR is binded with a milestone, if not, The Travis CI will output a warning message.
3. Check if your PR description contains keywords Documentation and it's correponding http links.if not, The Travis CI will output a warning message.
4. Check if your PR description contains keywords Demo and it's correponding http links.if not, The Travis CI will output a warning message.
5. update the changelog check rule message

Checklist

• [] Demo
• Documentation:add pr static check rules incubator-weex-site#447

[YorkShen]

FYI: You can use markdown function in danger

[wrmswindmill]

use markdown function is more readable. I have updated the code

[YorkShen]

LGTM

[YorkShen]

The check rules are described in https://github.com/apache/incubator-weex/blob/master/CONTRIBUTING.md#publish-your-change