Add gamma(::Complex{BigFloat})

[MikaelSlevinsky]

Current output on v2.2.0:

julia> using SpecialFunctions

julia> gamma(big(1)+im)
ERROR: MethodError: no method matching _loggamma(::Complex{BigFloat})
Closest candidates are:
  _loggamma(::Number, ::Number) at ~/.julia/packages/SpecialFunctions/gXPNz/src/gamma_inc.jl:1175
  _loggamma(::ComplexF16) at ~/.julia/packages/SpecialFunctions/gXPNz/src/gamma.jl:522
  _loggamma(::ComplexF32) at ~/.julia/packages/SpecialFunctions/gXPNz/src/gamma.jl:522
  ...
Stacktrace:
 [1] loggamma(x::Complex{BigFloat})
   @ SpecialFunctions ~/.julia/packages/SpecialFunctions/gXPNz/src/gamma.jl:634
 [2] _gamma(z::Complex{BigFloat})
   @ SpecialFunctions ~/.julia/packages/SpecialFunctions/gXPNz/src/gamma.jl:589
 [3] gamma(x::Complex{BigInt})
   @ SpecialFunctions ~/.julia/packages/SpecialFunctions/gXPNz/src/gamma.jl:567
 [4] top-level scope
   @ REPL[28]:1

julia> 

[hm271]

I have the same problem with v2.3.1. It would be very nice if gamma(::Complex{BigFloat}) could be implemented.