Add FillPathTemplateString methods to AtataContext

[YevgeniyShunevych]

Add methods to AtataContext, which are similar to FillTemplateString but additionally the variables are sanitized for path by replacing invalid characters with '_':

public string FillPathTemplateString(string template);

public string FillPathTemplateString(string template, IDictionary<string, object> additionalVariables);

Also add core method to TemplateStringTransformer:

public static string TransformPath(string template, IDictionary<string, object> variables);