Add side_effect option to fake files

[simonfagerholm]

Background:
On linux, parts of the file system have other uses than being actual files. One example is "/sys/class/gpio".
To mock the expected behaviour in this example a "side effect" of writing to the files is needed:

1. When writing an int XX to "/sys/class/gpio/export" a new directory is created "/sys/class/gpio/gpioXX" with some files
2. when writing an int XX to "/sys/class/gpio/unexport" the directory "/sys/class/gpio/gpioXX" is removed.

This "side effect" needs to happen in the same way during testing as in real environment to not affect the SUT. For example, if the folder "/sys/class/gpio/gpioXX" is created in the setup of the test the SUT can see that gpioXX is already available and skip writing to "/sys/class/gpio/export".
Another solution could have been polling the files if export has been written to, however this problems as the polling must be much faster than the SUT the be able to cause the side_effect to simulate the real environment.

Pull request:
The proposed solution in this pull request is to add a side_effect argument to fake files which can contain a function handle that is invoked, with the file object as argument, after the file is written. This function then causes the "side effects" needed.
The default is to not have any "side effect", preserving legacy.
I also added 2 unittests for the new functionality.

[mrbean-bremen]

Thanks for that - looks good to me!
@jmcgeheeiv - can you have a look and merge if it is ok?

[jmcgeheeiv]

Thank you for this, @simonfagerholm. This expands pyfakefs in a whole new direction.