Allow one-liner match statements

[MasonProtter]

Closes #93.

julia> using MLStyle

julia> @match 1 x => x + 1
2

I also updated the docs on pattern matching to include a note about the strucutre of @match and this syntax.

[codecov]

Codecov Report

Merging #96 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #96   +/-   ##
=======================================
  Coverage   90.34%   90.34%           
=======================================
  Files          28       28           
  Lines        1336     1336           
=======================================
  Hits         1207     1207           
  Misses        129      129           

Impacted Files	Coverage Δ
src/MatchImpl.jl	95.00% <100.00%> (ø)

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[thautwarm]

I'll merge it after I get up.