Feature rbenv gemset options

### Added option file and option parsing:
  - rbenv-gemset now can use an option file `.rbenv-gemset-options`
  - the option file can contain options in the following form:
```
this-is-an-option=value
this-is-another-option=another_value
```
  - new command: `rbenv-gemset-optionfile` which returns an option file (similar to rbenv-gemset-file)
  - new command: `rbenv-gemset-option <optionname>` which returns the value of the option if set in an option file, if option is not set, it will return an empty string

### Added options which can be set in option file:
  - `disable-global-gemset=yes` this will disable the automatic usage of the `global` gemset
  - `exclude-default-gempath=yes` this will disable the usage of the default gem path, resulting in an minimal gemset and using only the gems included in your Ruby and gemsets you added via rbenv-gemset