Fix empty/null/undefined values for nested objects in filter/args

[jakubknejzlik]

This PR contains:

• update code with prettier package (please let me know, if it's a problem)
• updates parsing filter/args to handle nested objects and empty values (https://github.com/codemeasandwich/graphql-query-builder/issues/5)
• possibly fixes undefined values by omitting them (https://github.com/codemeasandwich/graphql-query-builder/issues/7)

[coveralls]

Coverage remained the same at 100.0% when pulling 9d95a8a on jakubknejzlik:master into 52c12a8 on codemeasandwich:master.