'modifiable' boolean option

The main purpose of this pull request is to add a local 'modifiable' option to every editor to make it (un)modifiable. If this options is unset (false), Vrapper will make no changes to the document and print an error to the status line.

If an Eclipse editor reports that it is not editable, another error message will be printed and the 'modifiable' flag will be treated as unset in any case.

If 'syncmodifiable' is set (the default), the editor will automatically set the 'modifiable' option when the Eclipse editor is marked as read-only. This setting is a workaround for the fact that Vim has no real autocommands and functions, as Vim users could use the function in http://vim.wikia.com/wiki/Make_buffer_modifiable_state_match_file_readonly_state to do this.