Specify columns as strings?

[devmotion]

Related to #43, I noticed that columns can't be specified as strings:

julia> table_one(pkpain_df, names(pkpain_df); show_n=true)
ERROR: MethodError: no method matching SummaryTables.Analysis(::String, ::DataFrame)

Closest candidates are:
  SummaryTables.Analysis(::Any, ::Any, ::Any)
   @ SummaryTables ~/.julia/packages/SummaryTables/WKLcB/src/table_one.jl:82
  SummaryTables.Analysis(::Symbol, ::DataFrame)
   @ SummaryTables ~/.julia/packages/SummaryTables/WKLcB/src/table_one.jl:87
  SummaryTables.Analysis(::Symbol, ::Any, ::DataFrame)
   @ SummaryTables ~/.julia/packages/SummaryTables/WKLcB/src/table_one.jl:96
  ...

Stacktrace:
 [1] (::SummaryTables.var"#32#33"{DataFrame})(x::String)
   @ SummaryTables ~/.julia/packages/SummaryTables/WKLcB/src/table_one.jl:117
 [2] iterate
   @ ./generator.jl:47 [inlined]
 [3] _collect
   @ ./array.jl:854 [inlined]
 [4] collect_similar
   @ ./array.jl:763 [inlined]
 [5] map
   @ ./abstractarray.jl:3285 [inlined]
 [6] make_analyses
   @ ~/.julia/packages/SummaryTables/WKLcB/src/table_one.jl:117 [inlined]
 [7] table_one(table::DataFrame, analyses::Vector{…}; groupby::Vector{…}, show_overall::Bool, show_pvalues::Bool, show_tests::Bool, show_confints::Bool, show_n::Bool, compare_groups::Vector{…}, nonnormal::Vector{…}, minmax::Vector{…}, tests::@NamedTuple{…}, combine::MultipleTesting.Fisher, sort::Bool, celltable_kws::@Kwargs{})
   @ SummaryTables ~/.julia/packages/SummaryTables/WKLcB/src/table_one.jl:263
 [8] top-level scope
   @ REPL[120]:1
Some type information was truncated. Use `show(err)` to see complete types.

In my use case this would have been convenient (arguably names(df) is a bit simpler than propertynames(df)).

[jkrumbiegel]

Good point, I'll add the option to use strings as well. When you have column names with spaces (I guess you shouldn't but...), symbols are also not easy to use.