Allow user to specify generic loss function to do SGD on

[dustinvtran]

This cannot be efficiently implemented in Rcpp. Rcpp cannot interface with the R function in any way other than calling it in R each time. Hence for now, we are using strings to match models specified by the user, which allows for faster computation using C++ functions. Later on, we will extend this where some model argument in SGD can either be a string (uses C++ function on case-by-case basis) or a generic function (uses R function).

[dustinvtran]

Look into how optim() and gmm() do this, since they're quite fast but also allow users to specify a generic function.

[lantian2012]

The optim() and lapply() in the built in r core package evaluates the user defined functions in R by calling the function in R. For now, we can also call the function in R with sacrifice in performance.