Re-define `~`: macro => function

[yebai]

Julia 0.6 stopped supporting parsing ~ as a macro by default. We might need to perform this parsing in the model macro on our own and redefine ~ as a function.

[xukai92]

Comments from other developer: syntax sugar like @model and ~ is cool and good for end users but we should also make the plain function available (at least have a section "For Developers" to explain how to use them directly)

[yebai]

Relevant discussions on this matter:

JuliaLang/julia#4882

JuliaStats/GLM.jl#116

[kleinschmidt]

StatsModels.jl handles ~ in a Formula as a function now: see JuliaStats/StatsModels.jl#9

[yebai]

@kleinschmidt Thanks, I'll take a look at that. If you know how to port it to Turing, please get in touch.