Add support for multiline string literals

[spaceweasel]

Currently, an env variable can hold a multiline string, for example:

func main() {
	env := map[string]interface{}{
		"lines": `Here are
two lines`,
	}

	out, _ := expr.Eval("lines", env)
	fmt.Print(out)
}

Which prints out:

Here are
two lines

Whereas if I want to use a multiline string literal, then an error is returned.

	out, err := expr.Eval("'Here are\ntwo lines'", env)
	if err != nil {
		panic(err)
	}

panic: literal not terminated (2:1)
 | two lines'
 | ^

A workaround is to use double backslashes:

	out, err := expr.Eval("'Here are\\ntwo lines'", env)

Whilst this gives the desired output in this situation, it is impractical for our case.
Apart from the excessive line length, there are other issues like readability.

We have a requirement where the input strings do not exist in Go code, but are read from other sources (e.g. yaml files) and use the map function on a collection. An example would be something like this, but with many more lines and arguments:

map(things, sprintf(`
Thing Name: %s
Thing Size: %d`, 
.name, 
.size))

This PR adds support for multiline string literals through the backtick character, so it will handle our situation as well as more simple ones:

	out, err := expr.Eval("`Here are\ntwo lines`", env)

[antonmedv]

I'd like to use ` char for raw strings. Now only multiline strings. Let's implement those instead.

[antonmedv]

Tests for raw strings.

[antonmedv]

Implemented in #485