Provide an option to ignore empty/falsy objects/arrays/values in encodeQString

[bengillies]

Currently, encoding empty/falsy objects/values as a query string results in params that represent empty versions of that data type. For example:

> juri.encodeQString({
  foo: {},
  bar: [],
  baz: '',
  qux: false
});

'foo=(:)&bar=()&baz=\'\'&qux=--'

I can can see how this might be useful in some cases, but in other cases, it's both shorter and more reasonable to just ignore them completely. Ideally, something along the following lines would be great:

> juri.encodeQString({
  foo: {},
  bar: [],
  baz: '',
  qux: false
}, { stripEmpty: true });

''