Add source code.

include/ExternalEditor.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace ExternalEditor {
+
+// $VISUAL, then $EDITOR, then the first of nano/vim/vi found on PATH.
+// Empty if nothing was found.
+std::string defaultCommand();
+
+// Is the first word of `command` an executable we can find?
+bool commandExists(const std::string& command);
+
+// Well-known editors that are installed (used for tab completion).
+std::vector<std::string> installedEditors();
+
+// Opens `initialText` in the editor and returns the edited text.
+// Throws std::runtime_error if the editor cannot be run or exits with an error.
+std::string edit(const std::string& command, const std::string& initialText);
+
+}  // namespace ExternalEditor