✨ Add ability to download nerdfont symbols (#10)

Same as the other symbol download options, this one takes a list of all
nerdfont icons and wrangles them into shape to be usable with the emoji
picker. Can be used by invoking `bemoji -D nerd` or `bemoji -D all`.

Fixes #10.

test/bemoji_download.bats

@@ -38,3 +38,11 @@ emoji2 picked up"
     assert_equal "$outcome" "Σ GREEK CAPITAL LETTER SIGMA"
     unstub curl
 }
+@test "Runs nerdfont download on -D nerd option" {
+    stub curl \
+        "printf 'meangingless\nafiller lines\n.nf-md-pipe_wrench:before { \n content: \"\\\f1354\";\n }'"
+    run bemoji -D nerd 3>&-
+    outcome=$(cat "$BEMOJI_DB_LOCATION/nerdfont.txt")
+    assert_equal "$outcome" "󱍔 md-pipe_wrench"
+    unstub curl
+}