CLT-213 Add -e option to support other backends

Changes:
* new command `td server:endpoint` configures the default API server
  and stores it on the configuration file (td.conf);
* the -e / --endpoint option allows specifying an API server on-the-fly.
  The option is valid only for the duration of the command and is not
  saved. The `td account` command is an exception, see below;
* the `td -e / --endpoint <endpoint> account` command
  stores the endpoint in the configuration file (td.conf) as well
  for convenience and single sign-on;
* the -e / --endpoint option takes precedence over TD_API_SERVER
  environment variable;
* -e / --endpoint option is shown in command helpers (as for
  the -k / --key option);