update extra_pylint_args setting for list input

[bytesofmyself]

This was probably the original intention of this setting but currently it only allows one argument to be passed as a string. More than one argument causes both arguments to fail unless the first argument ends in a comma and even then any additional argument fails.

Works:
"pylint_extra": "--ignored-classes=MsgProto"

Both args fail:
"pylint_extra": "--ignored-classes=MsgProto --extension-pkg-whitelist=lxml,cassandra"

First arg works:
"pylint_extra": "--ignored-classes=MsgProto, --extension-pkg-whitelist=lxml,cassandra"

This setting should instead a list of strings instead of a string
"pylint_extra": ["--ignored-classes=MsgProto", "--extension-pkg-whitelist=lxml,cassandra"]