add option (per server) to send a keep alive signal with a user defined time interval

[ghost]

Many systems will timeout the SFTP session leaving the mount broken after a certain amount of time has passed. The workaround currently is to hit refresh in the ChromeOS File Manager before this happens. However, it would be even better if the app performed a keep alive of its own at a user defined time interval.

[cfrazier]

Or, when starting a new operation, simply check for the connection and reestablish it when needed.

[yoichiro]

The pull request #136 has already been merged into the master branch to reconnect to the connected SFTP server when the connection disconnected.