Object properties can be merged instead of overwrite

[tkow]

Check this after #31.

This is for such as media parameter can be value objects.
If using overwrite,

{ 
  "@bp": {
      width: '20px',
   },
  "someUtilFunc":  true
  // suppose generating "@bp": {
  //      height: 20px,
  // }, 
}

will omit width with util function because the latter overwrites the former.
Furthermore,

styled("View", { 
  "@bp": {
      width: '20px',
   }
},
{
   "@bp": {
      height: '20px',
   }, 
})

is same result and it looks bug as I expected. I fix it to get result the followed by in both case with some refactoring to make code readable.

{
  "@bp": {
     width: 20px,
     height: 20px,
  }, 
}

[tkow]

It's merged with #33.