WIP: Low severity threshold

[ernilambar]

Fixes #691

• Introduce flag in CLI for including low severity errors.
• Behat test added for testing new flag

[ernilambar]

Example:
Before with no severity argument

FILE: pc-sample.php
type,code,severity
ERROR,allow_unfiltered_uploads_detected,7
ERROR,plugin_header_non_functional_plugin_uri,7
ERROR,localhost_code_detected,8
ERROR,obfuscated_code_detected,6
ERROR,WordPress.WP.I18n.MissingTranslatorsComment,5
ERROR,WordPress.WP.I18n.TextDomainMismatch,5
ERROR,WordPress.WP.I18n.NonSingularStringLiteralDomain,5
ERROR,WordPress.Security.EscapeOutput.OutputNotEscaped,5

With --error-severity=7:

FILE: pc-sample.php
type,code,severity
ERROR,allow_unfiltered_uploads_detected,7
ERROR,plugin_header_non_functional_plugin_uri,7
ERROR,localhost_code_detected,8

With --error-severity=7 --include-low-severity-errors:

FILE: pc-sample.php
type,code,severity
ERROR,allow_unfiltered_uploads_detected,7
ERROR,plugin_header_non_functional_plugin_uri,7
ERROR,localhost_code_detected,8
ERROR_EXTRA,obfuscated_code_detected,6
ERROR_EXTRA,WordPress.WP.I18n.MissingTranslatorsComment,5
ERROR_EXTRA,WordPress.WP.I18n.TextDomainMismatch,5
ERROR_EXTRA,WordPress.WP.I18n.NonSingularStringLiteralDomain,5
ERROR_EXTRA,WordPress.Security.EscapeOutput.OutputNotEscaped,5