Added a way to set speaking rate in speech method

# Overview
Adds a way to set the speaking rate (wpm) in the speech method without having to manually use the applescript method

# Examples

```ruby
speak "HELLO!", "Alex", 250 # Says "HELLO!" with voice "Alex" at a speaking rate of 250 wpm
```
```ruby
speak "HELLO!" # Says "HELLO!" with voice "Alex" at a default speaking rate of 175 wpm
```

# Changes
- [x] Added speech rate parameter to method speech
- [x] Updated documentation to reflect changes
- [x] Bumped version to 4.1.4 (from 4.1.3)