Ability to specify what runs when starting tasksh

[boughtonp]

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...