Users have indicated that it will be very useful if the plugin exposes a
way to detect memory leaks in tests. This is possible, but is a bit
tricky as the interpreter can allocate memory for internal caches, as
well as user functions.

To make this more reliable, the new marker will take two parameters:

* The limit of memory per location to consider an allocation. If the
  memory leaked by any allocation location in the test is higher than
  this value, the test will fail.

* An optional callable function that can be used to filter out
  locations. This will allow users to remove false positives.

Signed-off-by: Pablo Galindo <[email protected]>

Signed-off-by: Matt Wozniski <[email protected]>