combine: `hcat` support

[iblislin]

For simple use case, it's faster than merge. (merge serves as inner / outer join, which differs from hcat)

julia> @benchmark merge($cl, $ohlc)
BenchmarkTools.Trial:
  memory estimate:  210.75 KiB
  allocs estimate:  5122
  --------------
  minimum time:     273.470 μs (0.00% GC)
  median time:      279.141 μs (0.00% GC)
  mean time:        299.841 μs (5.72% GC)
  maximum time:     2.998 ms (79.71% GC)
  --------------
  samples:          10000
  evals/sample:     1

julia> @benchmark [$cl $ohlc]
BenchmarkTools.Trial:
  memory estimate:  58.14 KiB
  allocs estimate:  1360
  --------------
  minimum time:     77.604 μs (0.00% GC)
  median time:      80.881 μs (0.00% GC)
  mean time:        102.880 μs (6.02% GC)
  maximum time:     5.132 ms (90.05% GC)
  --------------
  samples:          10000
  evals/sample:     1

[coveralls]

Coverage increased (+0.2%) to 86.364% when pulling 1044400ff127ae8fe74a0a5c3db57ac91ad2f369 on iblis17:hcat into 356cab1c7d614f03fa446c1483315adf1ea14578 on JuliaStats:master.

[iblislin]

conflicting resolved.

[coveralls]

Coverage increased (+0.2%) to 86.304% when pulling 5a6095bdaa4fb1b60d85cc996937e71d895458c2 on iblis17:hcat into 2cb954ea6437084bbe0938bee52d56133d2b02b9 on JuliaStats:master.