app_morsecode: Add American Morse code

Previously, the Morsecode application only supported international
Morse code. This adds support for American Morse code and adds an
option to configure the frequency used in off intervals.

Additionally, the application checks for hangup between tones
to prevent application execution from continuing after hangup.

ASTERISK-29541

Change-Id: I172431a2e18e6527d577e74adfb05b154cba7bd4