add peaceant's port of gruvbox themes

This adds Martin Radimec's [1] port [2] of the gruvbox theme [3].

Brian Reilly (Briles) [4] has a pretty complete collection of gruvbox
ports [5] but they are all in `sublime-color-scheme` which is not yet
supported in bat, see #759 [6].

[1] https://github.com/peaceant
[2] https://github.com/peaceant/gruvbox
[3] https://github.com/morhetz/gruvbox
[4] https://github.com/Briles
[5] https://github.com/Briles/gruvbox
[6] https://github.com/sharkdp/bat/issues/759

.gitmodules

@@ -185,3 +185,6 @@
 [submodule "assets/syntaxes/02_Extra/AsciiDoc"]
 	path = assets/syntaxes/02_Extra/AsciiDoc
 	url = https://github.com/asciidoctor/sublimetext-asciidoc.git
+[submodule "assets/themes/gruvbox"]
+	path = assets/themes/gruvbox
+	url = https://github.com/peaceant/gruvbox.git

CHANGELOG.md

@@ -11,6 +11,9 @@
 - Update AsciiDoc syntax, see #1034 (@rxt1077)
 
 ## New themes
+
+- Gruvbox, see #1069 (@kyleondy)
+
 ## `bat` as a library
 
 - Add APIs to provide `Input` descriptions with `InputDescription` (@eth-p)