Update table component to support borderless and sizing props

[stevesims]

It's now possible to set a borderless flag on the Table component to make the borders vanish.

Additionally the vertical sizing of cells (padding) can be set by specifying a sizing prop. By default this uses the value of SPACING.SCALE_3 from @govuk-react/constants

[codecov]

Codecov Report

Merging #109 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #109   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          27     27           
  Lines         215    218    +3     
  Branches       46     46           
=====================================
+ Hits          215    218    +3

Impacted Files	Coverage Δ
components/table/src/index.js	100% <100%> (ø)	⬆️

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[gavinorland]

We can work on the skipped test, as discussed :)

[gavinorland]

I found out a bit more about this: if either the first-child or the last-child blocks are present, then the first style property outside them is set but cannot be found, regardless of what it is and of whether those blocks are placed before of after the other rules. So, for example, if we move borderBottom to after display then it's found, but display isn't!