Declare & implement space_around_attr_eq config.

This configuration setting controls the spacing around `=` when found
inside of attributes. Eg:

```rust
 #[cfg(target_os="linux")]
 // vs
 #[cfg(target_os = "linux")]
```

The config setting `space_around_attr_eq` takes a bool value.

Closes #4039

Author: thedodd

rustfmt-core/rustfmt-config/src/lib.rs

@@ -73,6 +73,8 @@ create_config! {
         "Determines if '+' or '=' are wrapped in spaces in the punctuation of types";
     space_before_colon: bool, false, false, "Leave a space before the colon";
     space_after_colon: bool, true, false, "Leave a space after the colon";
+    space_around_attr_eq: bool, true, false,
+        "Determines if '=' are wrapped in spaces in attributes.";
     spaces_around_ranges: bool, false, false, "Put spaces around the  .. and ..= range operators";
     binop_separator: SeparatorPlace, SeparatorPlace::Front, false,
         "Where to put a binary operator when a binary expression goes multiline";

rustfmt-core/rustfmt-lib/src/attr.rs

@@ -252,7 +252,11 @@ impl Rewrite for ast::MetaItem {
                 // See #2479 for example.
                 let value = rewrite_literal(context, literal, lit_shape)
                     .unwrap_or_else(|| context.snippet(literal.span).to_owned());
-                format!("{} = {}", path, value)
+                if context.config.space_around_attr_eq() {
+                    format!("{} = {}", path, value)
+                } else {
+                    format!("{}={}", path, value)
+                }
             }
         })
     }