Add ShellForEach method for running per-line commands via the system shell

[ChrisJr404]

Following on from the discussion in #184, this adds ShellForEach, the per-line companion to the new Shell method.

ExecForEach splits each rendered command with shell.Fields, so a template value containing spaces gets broken up into separate arguments (the touch {{.}} on a filename with spaces problem in that issue). ShellForEach passes each rendered line to the system shell instead, so you can quote the value and let the shell deal with it:

script.ListFiles("*").ShellForEach("touch '{{.}}'").Wait()

Under the hood it runs each line through sh -c (cmd /C on Windows) the same way Shell does, and the error handling, environment, and context behaviour all match ExecForEach. Added the docs, an example, and tests including the quoted-argument case.