Encoding functions

[davibarreira]

Resolves #16.

Implements possibility of passing functions in the plot function:

data = StructArray(x=range(0,10,20), y=range(0,10,20))
plt = plot(
    data,
    x=(field=:x, datatype=:q),
    y=row -> row.x^2,
)
draw(plt)

plt = plot(
    data,
    x=(value=collect(range(0, 20, 20)), datatype=:q),
    y=(value=row -> row.x^2,),
    color=(field=:z,),
)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.

Project coverage is 66.34%. Comparing base (bd2b3af) to head (2a04609).

Files	Patch %	Lines
src/encoding/quickplot.jl	82.14%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #17 +/- ## ========================================== + Coverage 65.23% 66.34% +1.11% ========================================== Files 70 70 Lines 2229 2255 +26 ========================================== + Hits 1454 1496 +42 + Misses 775 759 -16 ```

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