[TW-1829] Optional include statement variant

_Georg Sauthoff on 2016-07-11T19:49:34Z says:_

When using a master `~/.taskrc` with different users it seems to be useful to include a `~/.taskrc.local` for truly user specific changes.

Thus, for this use case a variant of the include statement would be nice that does nothing if the to be included file is not found (i.e. no error).

Similar to GNU make which has `include` and `-include`.

Sure, alternatively one can just touch such a `~/.taskrc.local` file such that it is present, but empty.