The `sh.Exec` family of functions used to use `os.Expand` to expand
environment variables in command arguments, but that function offers no
escape syntax -- no way to avoid expanding dollar signs that the caller
really wants to include on the command line. This commit implements a
new, mostly compatible `Expand` function to allow escaping the
environment-variable syntax, plus an accompanying `Escape` function to
producing escaped strings.

This technically breaks backward compatibility; any existing code that
has used the sequence "\\" or "\$" in its command arguments will need to
be updated. Although the compatibility argument prevents `os.Expand`
from changing, documentation for `sh.Exec` hasn't actually stated that
it follows all the same rules as `os.Expand`, so it's possible to view
this change as merely a bugfix, formalizing a new syntax that had not
previously been established.

Fixes magefile#434