Allow showing the expanded macros of only a given module / source file

When working with many macros, looking at the expanded macro output of the whole crate doesn't scale.

There should be an option to only show the expanded macros from a given module / source file.

E.g. in one case I have 81733 lines in the expanded macro output from cargo rustc -- -Z unstable-options --pretty=expanded...

1. I should have reported this when https://github.com/dtolnay/cargo-expand/issues/8 was filed, but :+1: from me and :+1::+1::+1::+1::+1::+1::+1::+1::+1::+1::+1::+1: from 12 people over there.

   David Tolnay at 2017-11-19 08:52:39

2. I implemented a workaround in cargo-expand 0.4 for now by filtering the compiler's output through Syn. For example you can run:

   expand contents of one module:

   $ cargo expand punctuated::printing
   

   expand a single item (struct, function, trait, etc):

   $ cargo expand token::FatArrow
   

   David Tolnay at 2019-01-14 07:42:45