Convenient methods to specify required local commands

[vladimirvivien]

Echo.Prog.Avail(path) can be used to determine if a command is available on the specified path. However, as an echo user, it would be nice to make such assertment as follow:

e := echo.New().RequiresCmd(path)
e.Run(<command>)

Echo would validate that the required command is on the provided path and fails if not found.