Parse shell-like line continuation in makeflow files
Merge branch 'andreyto-at_makefow_line_cont'

Per Andrey:

Sometimes I write makeflows by hand instead of generating them,
and prefer the makeflows to be readable.
In those cases, it is convenient to split long commands across
multiple lines using backslash at the end of the line.
This branch implements parsing and provides the test case.

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This commit does not handle newline escapes at the end of comment lines.