Improve post! and DID result interface and add GroupContrasts

[junyuan-chen]

All subtypes of AbstractDIDResult now have a parameter for treatment type.
The vector of StatsSpecs constructed with @specset should always be returned.

[codecov]

Codecov Report

Merging #33 (ffe3f73) into master (ba1abb8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #33   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files          12       12           
  Lines        1378     1416   +38     
=======================================
+ Hits         1376     1414   +38     
  Misses          2        2           

Impacted Files	Coverage Δ
src/DiffinDiffsBase.jl	100.00% <ø> (ø)
src/StatsProcedures.jl	99.30% <100.00%> (+<0.01%)	⬆️
src/did.jl	100.00% <100.00%> (ø)
src/procedures.jl	100.00% <100.00%> (ø)
src/tables.jl	100.00% <100.00%> (ø)

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