Sync editorconfig and rustfmt (vercel#352)

I'm using the [default](https://rust-lang.github.io/rustfmt/?version=v1.5.1&search=#max_width) `100` line length from rustfmt, but making it explicit.

Author: jridgewell

.editorconfig

@@ -11,6 +11,10 @@ tab_width = 4
 insert_final_newline = true
 max_line_length = 120
 
+[*.rs]
+# Keep in sync with rustfmt
+max_line_length = 100
+
 [{*.markdown,*.md}]
 max_line_length = 80
 

.rustfmt.toml

@@ -1,3 +1,6 @@
+# Keep in sync with .editorconfig
+max_width = 100
+
 tab_spaces = 4
 hard_tabs = false