toast should add comment specifying cmd that was used to generate nim file
analog to what nim does to C files eg:
```c
/* Generated by Nim Compiler v0.19.9 */
/*   (c) 2018 Andreas Rumpf */
/* The generated code is subject to the original license. */
/* Compiled for: MacOSX, amd64, clang */
/* Command for C compiler:
   clang -c  -w -g  -I/Users/timothee/git_clone/nim/Nim/lib -I/Users/timothee/git_clone/nim/timn/tests/nim/all -o /tmp/foo/bar.c.o /tmp/foo/bar.c */
```


it could show all relevant info eg:
```
generated at D20190124T190319 (or other human+machine friendly format for timestamp)
# replace w full path to toast?
toast --pnim --preprocess --defines+=FORCE --includeDirs+=/private/tmp/d34/nimterop/tests/include /private/tmp/d34/nimterop/tests/include/test.h
```
