`--profile-svg` doesn't work on windows, fails silently

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`pytest-profile` uses [`pipes.py`](https://docs.python.org/2/library/pipes.html) to create a pipe that converts a `.prof` file to an `.svg` using external programs, but `pipes.py` only produces posix-shell commands. The pipe is defined by `pytest-profile` as follows:

t = pipes.Template()
t.append("gprof2dot -f pstats $IN", "f-")
t.append("dot -Tsvg -o $OUT", "-f")
t.copy(self.combined, self.svg_name)

At the last line of the above snippet, `pipes.py` calls `os.system( str_command )` with `str_command` being the following literal:

IN=''; gprof2dot -f pstats $IN |
{ OUT=''; dot -Tsvg -o $OUT; }

...which isn't valid Windows batch syntax.

Start with the pytest-profile implementation of --profile-svg and the pipes.py usage shown in the issue. Trace how the .prof-to-.svg command is assembled and executed on Windows; done means --profile-svg produces the SVG successfully instead of failing silently.

Written by the indexing model from the issue text.