vcs: Restructure vcs module

Just like writing and qutebrowser modules, restructured the version
control software module to make more use of dotter's ability to
precisely link files. All contained programs have a top-level directory
and all the files that correspond to that specific software lie beneath
in the directory tree.

vcs/gitignore/config/zsh/completions/_gitignore

@@ -0,0 +1,15 @@
+#compdef _gitignore gitignore
+#
+# Requires gitignore script in path
+#
+# Enables completion for zsh of gitignore function.
+
+_gitignore_get_command_list() {
+  curl -sL https://www.gitignore.io/api/list | tr ',' '\n'
+}
+
+_gitignore() {
+  _arguments \
+    '1::flags:((-f\:"Save output to .gitignore file in current directory"))' \
+    ":listopts: _values -s, 'modules' $(_gitignore_get_command_list)"
+}