feat: add function to extends/override builtin functions

[Blackman99]

This PR provide ability to extends/override builtin functions.

API explanation

jmespath.extendsFunction('functionName', function (supportedTypes) {
	return {
		// the function implemention
		processor: function (resolvedArgs) {
			// do somthing with args and return the expected result
		},
		// the signature for validating function parameters
		signature: [
			{
				types: [
					supportedTypes.TYPE_NUMBER,
					// more types...
				]
			}
		]
	}
})

Example usage

 jmespath.extendsFunction('foo', function (supportedTypes) {
	return {
		processor: function (resolvedArgs) {
			return 99 + resolvedArgs[0];
		},
		signature: [
			{ types: [supportedTypes.TYPE_NUMBER] }
		]
	}
});
jmespath.search({ bar: 1 }, 'foo(bar)')
// The result should be 100

The new added extendsFunction function should has no side effects. It only collect built-in types into a object and provide it for custom function creator.

Tests

Relative test is included in this PR.

This PR should fix #25 #30