phpcs: support multiple standards

[jonhattan]

phpcs does support multiple standards (--standard=PEAR,PSR2). See https://github.com/squizlabs/PHP_CodeSniffer/wiki/Usage#specifying-a-coding-standard, but phpqa limits this to only one.

I'll prepare a PR to allow for multiple standards.

[zdenekdrahos]

Quickfix: multiple standards are supported in XML file.

<!-- phpcs.xml -->
<?xml version="1.0"?>
<ruleset name="ACME">
    <description>The coding standard for ACME PHP source code</description>

    <rule ref="PSR1" />
    <rule ref="PSR2" />

    <rule ref="PSR1.Classes.ClassDeclaration.MissingNamespace">
        <exclude-pattern>*/app/presenters/*.php</exclude-pattern>
    </rule>
    <rule ref="PSR1.Classes.ClassDeclaration.MultipleClasses">
        <exclude-pattern>src/Exceptions.php</exclude-pattern>
    </rule>
</ruleset>

# .phpqa.yml
phpcs:
    standard: phpcs.xml