unmarshal: support encoding.TextUnmarshaler

[oncilla]

This PR adds support for decoding fields of primitive types that implement
encoding.TextUnmarshaler by calling the custom method.

Fields in anonymous structs are not supported at this point.

fixes #329

[codecov]

Codecov Report

Merging #375 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #375      +/-   ##
==========================================
+ Coverage   95.09%   95.12%   +0.02%     
==========================================
  Files          10       10              
  Lines        2202     2214      +12     
==========================================
+ Hits         2094     2106      +12     
  Misses         68       68              
  Partials       40       40              

Impacted Files	Coverage Δ
marshal.go	94.54% <100.00%> (+0.10%)	⬆️

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[oncilla]

@pelletier I rebased on top of master.
I don't really know how to improve the code coverage.

One miss I can see is from:

func (d *Decoder) unmarshalText(tval interface{}, mval reflect.Value) error { 
	...
	default:
		return fmt.Errorf("unspported type: %t", val)
	...

But that should never executed, if the parser does not have a bug. Should I just put a panic there?
I think writing a test that actually triggers this error path is pretty brittle and does not really add value.

I did not figure out where the second miss comes from.

[pelletier]

Looks good to me! Thanks for taking care of it. Sorry it took me a while to get to it!