Add ignore array support in Coalesce()

[JoshuaC215]

Closes #64

[JoshuaC215]

Actual coverage, for reference:

ok      github.com/qntfy/kazaam 0.011s  coverage: 93.2% of statements
ok      github.com/qntfy/kazaam/kazaam  0.009s  coverage: 54.5% of statements
ok      github.com/qntfy/kazaam/transform       0.010s  coverage: 89.9% of statements

[JoshuaC215]

Goveralls seems to only be running top level tests and ignoring the ones in transform/

[JoshuaC215]

@chilland reviewed

[JeanMertz]

Looking at the example, this change means I can't have a source JSON key called ignore, and use the coalesce transformation on that key, is that correct?

I don't have that use-case, but I was writing some extra transformations (such as move) and was considering how to design the spec API, and took this one as an example, which got me thinking about the fact that the current API design means any extra added spec option name is another name that you can no longer use in the source JSON.