Add support for valueless tag options

- Tag options can have no value. The value is set to boolean `true`.  
For example, `[video nocontrols]...[/video]` instead of `[video nocontrols=1]...[/video]` works.
- Exit other states to `OPTION_STATE_DEFAULT` instead of `OPTION_STATE_KEY` in `Parser::parseOptions()`. This is necessary to exit `OPTION_STATE_KEY` without a value (`case ' '`).