Add PHP static analyzers (of element types) to build checks?

[TysonAndre]

Similar to #83

Motivation:

• More likely that phpdoc will remain accurate when refactoring
  • Improved autocompletion and error detection in projects using tolerant-php-parser
• Detect potential bugs in new commits/PRs that may be undetected by unit tests

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There are several open source PHP static analyzers, such as https://github.com/phan/phan (which I maintain) and https://github.com/vimeo/psalm

• https://github.com/Microsoft/tolerant-php-parser/pull/176/files and https://github.com/Microsoft/tolerant-php-parser/pull/177/files were (the more obvious) issues detected using phan while I was working on a project to convert this to another AST format.

• An example of how this may be set up is TysonAndre@daa503d (example output at https://travis-ci.org/TysonAndre/tolerant-php-parser/jobs/277628702)
  (Both phan and psalm have ways to suppress categories of issues and individual issues)