Ability to specify what runs when starting tasksh

I don't want the help output when starting tasksh, but I do always run the same report straight away.

So in tasksh/src/main.cpp `welcome` I added a check for config `tasksh.default` and (if defined) it gets appended to `task ` and executed.

Then I defaulted `tasksh.welcome=header,default,help` in taskwarrior/src/Context.cpp and updated the logic in tasksh `welcome` to loop through and output the items specified. (Then updated my taskrc file to just be `header,default`.)

If interested let me know and I'll look at what's needed for docs/tests/etc and send a patch or pull request...