Adds support for custom marshalers through the encoding.TextMarshaler interface

[Fontinalis]

This PR adds a check to each built-in coerce function, so if the value implements the encoding.TextMarshaler interface, it'll be marshaled and converted from that, so custom types, like some nullable type from github.com/guregu/null would work correctly.

Closes #471

[Fontinalis]

Waiting for at least one review.. I could merge it if it would be someone else’s PR, but cannot do it without a review since it’s mine..

…

On 2019. Apr 15., at 15:23, billkillapp ***@***.***> wrote: Could we merge this feature ASAP? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or mute the thread.

[limoli]

When are you going to merge this important feature into master?

[coveralls]

Coverage decreased (-0.09%) to 92.288% when pulling b7c3a1a on custom-marshaler into 02caa89 on master.

[Fontinalis]

@chris-ramon Can you review this?