Summary: Adds a linter that checks php files for syntax errors using php -l
Test Plan:
Add a section to your .arclint file similar to:
```
"php": {
"type": "php",
"include": "(\\.php$)"
}
```
Then run arc lint on a php file with a syntax error.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley, jacobwalker0814, Korvin
Differential Revision: https://secure.phabricator.com/D10370
