Add two sample color schemes by Daniel Dettlaff

Thanks to Daniel Dettlaff, a.k.a. dmilith.
Add data/colors directory description to README
Convert eols of colorschemes for Windows deploy
Note addition of sample color schemes in ChangeLog
Merge branch 'add-sample-color-schemes'
Rename eval_echo() to eval_arglist()

Soon it will be used not only for :echo command.
Make :execute process arguments as expressions

This means that it acts very similar to the :echo command.
That's why common code was moved to a try_eval_arglist() function, which
is now called from echo_cmd() and exe_cmd().
Complete names of environment variables for :exe
Note in the documentation about :exe update
Merge branch 'enhance-exe-cmd'