Add parsing options, including option to skip automatic date parsing

[sebastien-rosset]

To support arbitrary date layout parsing, one approach would be to define a custom "parsetime" function, which would take a date/time layout as a parameter, with the same format as time.Parse():

	functions := map[string]govaluate.ExpressionFunction{
		"parsetime": func(args ...interface{}) (interface{}, error) {
			return time.Parse(args[0].(string), args[1].(string))
		},
	}

With the above function, it would be possible to evaluate the following expression:

parsetime('Mon Jan 2 15:04:05 2006', [ts])

where "ts" is a parameter. However, since govaluate always attempts to parse strings as dates, the layout ends up being parsed as a date.

I think it would be useful to provide a method to disable automatic date parsing, this may actually provide a small performance improvement as a side effect. I can work on a PR if that makes sense.