Add tilde boolean operator

This commit adds the new tilde '~' operator, which when used will
convert a value to a boolean before comparison.

For example, using the following JSON:

    {
      "vals": [
        { "a": 1, "b": true },
        { "a": 2, "b": true },
        { "a": 3, "b": false },
        { "a": 4, "b": "0" },
        { "a": 5, "b": 0 },
        { "a": 6, "b": "1" },
        { "a": 7, "b": 1 }
        { "a": 8, "b": "true" },
        { "a": 9, "b": false }
        { "a": 10, "b": null }
        { "a": 11 }
      ]
    }

You can now query for all true(ish) values:

    vals.#(b==~true)#

Which returns:

    [1,2,6,7,8]

Or all false(ish) values:

    vals.#(b==~false)#

Which returns:

    [3,4,5,9,10,11]

The last value which was non-existent is treated as "false"

gjson.go

@@ -1174,6 +1174,15 @@ func parseObject(c *parseContext, i int, path string) (int, bool) {
 }
 func queryMatches(rp *arrayPathResult, value Result) bool {
 	rpv := rp.query.value
+	if len(rpv) > 0 && rpv[0] == '~' {
+		// convert to bool
+		rpv = rpv[1:]
+		if value.Bool() {
+			value = Result{Type: True}
+		} else {
+			value = Result{Type: False}
+		}
+	}
 	if !value.Exists() {
 		return false
 	}

gjson_test.go

@@ -2064,3 +2064,25 @@ func TestEncodedQueryString(t *testing.T) {
 	assert(t, Get(json, `friends.#(last=="Mur\nphy").age`).Int() == 44)
 	assert(t, Get(json, `friends.#(last=="Murphy").age`).Int() == 47)
 }
+
+func TestBoolConvertQuery(t *testing.T) {
+	json := `{
+		"vals": [
+			{ "a": 1, "b": true },
+			{ "a": 2, "b": true },
+			{ "a": 3, "b": false },
+			{ "a": 4, "b": "0" },
+			{ "a": 5, "b": 0 },
+			{ "a": 6, "b": "1" },
+			{ "a": 7, "b": 1 }
+			{ "a": 8, "b": "true" },
+			{ "a": 9, "b": false }
+			{ "a": 10, "b": null }
+			{ "a": 11 }
+		]
+	}`
+	trues := Get(json, `vals.#(b==~true)#.a`).Raw
+	falses := Get(json, `vals.#(b==~false)#.a`).Raw
+	assert(t, trues == "[1,2,6,7,8]")
+	assert(t, falses == "[3,4,5,9,10,11]")
+}