Added basic syntax highlighting support for Rust

This commit adds basic syntax highlighting support for Rust (1):

Traits (2) and enums (3) are colorized with `nord7` and with bold font
to make them visually stand out more.
Also attributes (4) and derives (5) are colored with `nord10`.
Macros (6) are colorized with `nord8` and bold font to make them
visually different from "normal" functions.
Escape (7) sequences are colored with `nord13`.
Import statements and paths are correctly colored with keyword and type colors.

References:
  (1) https://www.rust-lang.org
  (2) https://doc.rust-lang.org/book/ch10-02-traits.html
  (3) https://doc.rust-lang.org/1.1.0/book/enums.html
  (4) https://doc.rust-lang.org/reference/attributes.html
  (5) https://doc.rust-lang.org/edition-guide/rust-2018/macros/custom-derive.html
  (6) https://doc.rust-lang.org/1.8.0/book/macros.html
  (7) https://doc.rust-lang.org/reference/tokens.html#ascii-escapes

GH-138

colors/nord.vim

@@ -451,6 +451,18 @@ hi! link rubyLocalVariableOrMethod Function
 hi! link rubyPseudoVariable Keyword
 hi! link rubyRegexp SpecialChar
 
+call s:hi("rustAttribute", s:nord10_gui, "", s:nord10_term, "", "", "")
+call s:hi("rustEnum", s:nord7_gui, "", s:nord7_term, "", "bold", "")
+call s:hi("rustMacro", s:nord8_gui, "", s:nord8_term, "", "bold", "")
+call s:hi("rustModPath", s:nord7_gui, "", s:nord7_term, "", "", "")
+call s:hi("rustPanic", s:nord9_gui, "", s:nord9_term, "", "bold", "")
+call s:hi("rustTrait", s:nord7_gui, "", s:nord7_term, "", s:italic, "")
+hi! link rustCommentLineDoc Comment
+hi! link rustDerive rustAttribute
+hi! link rustEnumVariant rustEnum
+hi! link rustEscape SpecialChar
+hi! link rustQuestionMark Keyword
+
 call s:hi("sassClass", s:nord7_gui, "", s:nord7_term, "", "", "")
 call s:hi("sassId", s:nord7_gui, "", s:nord7_term, "", s:underline, "")
 hi! link sassAmpersand Keyword