Update: keyword-spacing supports async/await (refs #7101)

[mysticatea]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[x] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Tell us about your environment

• ESLint Version: master
• Node Version: 6.3.0
• npm Version: 3.8.9

What parser (default, Babel-ESLint, etc.) are you using?

• default

Please show your full configuration:

• nothing

What did you do? Please include the actual source code causing the issue.

$ echo "({async'foo'() { }})" | eslint --stdin --no-eslintrc --parser-options "ecmaVersion:8" --rule "keyword-spacing:error"

What did you expect to happen?

• ESLint shows an error since there is no space preceded by the async contextual keyword.

What actually happened? Please include the actual, raw output from ESLint.

• No error.

Please check each item to ensure your pull request is ready:

• I've read the pull request guide
• I've included tests for my change
• I've updated documentation for my change (if appropriate)

What changes did you make? (Give an overview)

This PR fixes keyword-spacing rule to handle async/await contextual keywords.

Semver-minor: this is a bug fix which increases errors.

Is there anything you'd like reviewers to focus on?

There is the discussion about which rule should handle the space of between async and function-paren: #7101 (comment)
In this PR, keyword-spacing does not handle the space since I preferred consistency #7101 (comment)

[eslintbot]

LGTM

[nzakas]

Lgtm, just waiting another day.