Adding file deregister tool + extrapolating code

This solves some TODOs by moving shared code out into
`_blackbox_common.sh`.

New VCS commands were added, `vcs_ignore` and `vcs_notice` (the
opposite of ignore).

Made some utility functions

* `remove_filename_from_cryptlist` - The opposite of `add_file_to_cryptlist`
* `remove_line` - Removes a single line from a text file

bin/blackbox_deregister_file

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+
+#
+# blackbox_deregister_file -- Remove a file from the blackbox system.
+#
+# Takes an encrypted file and removes it from the blackbox system.  The
+# encrypted file will also be removed from the filesystem.
+
+set -e
+blackbox_home=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
+source "${blackbox_home}/_blackbox_common.sh"
+_determine_vcs_base_and_type
+
+unencrypted_file=$(get_unencrypted_filename "$1")
+encrypted_file=$(get_encrypted_filename "$1")
+
+if [[ "$1" == "$unencrypted_file" ]]; then
+  echo ERROR: Please only deregister encrypted files.
+  exit 1
+fi
+
+echo ========== PLAINFILE "$unencrypted_file"
+echo ========== ENCRYPTED "$encrypted_file"
+
+fail_if_not_exists "$encrypted_file" "Please specify an existing file."
+
+prepare_keychain
+remove_filename_from_cryptlist "$unencrypted_file"
+vcs_notice "$unencrypted_file"
+git rm "$encrypted_file"
+vcs_remove "$BB_FILES"
+
+vcs_commit "Removing from blackbox: ${unencrypted_file}"
+echo "========== UPDATING VCS: DONE"
+echo "Local repo updated.  Please push when ready."
+echo "    $(which_vcs) push"