Add lcfirst, ucfirst, ucwords to the basic string filters

Just adding the missing PHP functions for handling strings (the simple ones), with docs.

```
{{ 'HelloWorld'|lcfirst }}
{# outputs 'helloWorld' #}

{{ 'hello world!'|ucfirst }}
{# outputs 'Hello world!' #}

{{ 'hello world!'|ucwords }}
{# outputs 'Hello World!' #}
```

Any comments are appreciated.