Add an option to omit values equal to defaults on serialization

[Fatal1ty]

Is your feature request related to a problem? Please describe.
There are cases when it's preferable to omit default values on serialization. It can potentially reduce the storage space used or the amount of data transferred.

Describe the solution you'd like
Add new omit_default config and dialect option.

Describe alternatives you've considered
We can remove values equal to defaults using post-serialize hook but it's not as efficient.