produce escape sequences for extra Ctrl- modified keys

current xterm has the resource modifyOtherKeys which, when enabled,
constructs escape sequences for many Ctrl-, Shift-, and Meta- modified
keys.  This implements support for some Ctrl- modified keys.
provide a modify_other_keys config option.  defaults to false
Merge pull request #161 from holomorph/holomorph/modifyotherkeys

produce escape sequences for extra Ctrl- modified keys