Build fails on nightly (2018-04-26)

[CJStadler]

I confirmed that it builds on nightly-2018-04-19, which seems like it was the last nightly release. Here are the errors:

error[E0658]: procedural macros cannot be expanded to expressions
  --> maud_macros/src/build.rs:28:17
   |
28 |                 quote!($output_ident.push_str($string);)
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(proc_macro_non_items)] to the crate attributes to enable

error[E0658]: procedural macros cannot be expanded to expressions
  --> maud_macros/src/build.rs:60:19
   |
60 |           self.push(quote!({
   |  ___________________^
61 | |             // Create a local trait alias so that autoref works
62 | |             trait Render: maud::Render {
63 | |                 fn __maud_render_to(&self, output_ident: &mut String) {
...  |
68 | |             $expr.__maud_render_to(&mut $output_ident);
69 | |         }));
   | |__________^
   |
   = help: add #![feature(proc_macro_non_items)] to the crate attributes to enable

error[E0658]: procedural macros cannot be expanded to expressions
   --> maud_macros/src/build.rs:123:19
    |
123 |         self.push(quote!(if $cond { $body }));
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: add #![feature(proc_macro_non_items)] to the crate attributes to enable

error[E0658]: procedural macros cannot be expanded to expressions
  --> maud_macros/src/lib.rs:43:5
   |
43 | /     quote!({
44 | |         extern crate maud;
45 | |         let mut $output_ident = String::with_capacity($size_hint);
46 | |         $stmts
47 | |         maud::PreEscaped($output_ident)
48 | |     })
   | |______^
   |
   = help: add #![feature(proc_macro_non_items)] to the crate attributes to enable

[lambda-fairy]

Thanks for reporting!

Looks like it's just missing a feature flag. I'd be happy to accept a PR that adds it in.