Add support for cursor custom conversion

[fuelen]

Related to this: #61
This PR solves my issue with minimal changes
I've added the following lines to my application:

defimpl Paginator.Cursor.Encode, for: DateTime do
  def convert(term), do: {:dt, DateTime.to_unix(term, :microsecond)}
end

defimpl Paginator.Cursor.Decode, for: Tuple do
  def convert({:dt, unix_timestamp}), do: DateTime.from_unix!(unix_timestamp, :microsecond)
end

> datetime = DateTime.utc_now
#DateTime<2019-04-03 11:25:47.435854Z>

# before:
> Paginator.Cursor.encode([datetime])
"g2wAAAABdAAAAA1kAApfX3N0cnVjdF9fZAAPRWxpeGlyLkRhdGVUaW1lZAAIY2FsZW5kYXJkABNFbGl4aXIuQ2FsZW5kYXIuSVNPZAADZGF5YQNkAARob3VyYQtkAAttaWNyb3NlY29uZGgCYgAGpo5hBmQABm1pbnV0ZWEZZAAFbW9udGhhBGQABnNlY29uZGEvZAAKc3RkX29mZnNldGEAZAAJdGltZV96b25lbQAAAAdFdGMvVVRDZAAKdXRjX29mZnNldGEAZAAEeWVhcmIAAAfjZAAJem9uZV9hYmJybQAAAANVVENq"

# after:
> Paginator.Cursor.encode([datetime])                                                        
"g2wAAAABaAJkAAJkdG4HAE7r4IOehQVq"

The idea with custom cursor module from #9 looks more flexible, but there is no moving for a long time there, it is a bit outdated and I found it only during preparing this PR.

[sgerrand]

Hello @fuelen and thank you for creating this submission. Would you please rebase your changes against the main branch to resolve the existing conflicts.

[fuelen]

Hi @sgerrand

I can do this, but is it worth? Maybe it is better to go with #9?
I didn't close this PR in order to have a quick working solution for others and because some of my apps rely on this branch.

WDYT?

[sgerrand]

Maybe it is better to go with #9?

🙆 I'll try to bring that up-to-date later this week.