Dependency on fs

[Awnage]

Is there a way of doing a validation without needing the fs library? Given the README example it seems like it would be possible.
var results = validator.validate(xml, schematron);
both the xml and schematron variables are strings after toString().

[TimLuq]

My PR #2 resolves this issue by only loading readFile from fs if necessary. If you never use any file paths it will never import the library. It still has a guaranteed runtime dependency to the xpath library, though.

It is unclear from your comment if you're sending objects with a toString() method to validate() or you're sending strings (perhaps from earlier call to toString()) as parameters? The function call expects the parameters to already be strings and detects if it's a filepath/URL or serialized XML by checking if the first non-whitespace character is <.

After my PR is applied fs is imported when it detects a filepath, node-fetch is imported when it detects an URL and no fetch() is available in the global scope, and otherwise no library for loading external data is imported.