Fix for feature file encoding issue

* [Fix] After adding a non-ASCII in a sample feature, Gherkin raised an
encoding incompatibility error. Fixed
* [CHANGE] Added a new field
'feature_encoding' in Config object to store the encoding of feature
files (default = 'UTF-8')
* [NEW] Class `GherkinFacade`
* [CHANGE]
`Application#parse_features` method