Add `write_reference_line_numbers` option

[johantell]

Background: Since even the smallest changes in files can cause a cascade of changed line numbers in po/pot files we're often seeing larger amounts of changes in commits causing the need to re-extract than needed.

By adding the write_reference_line_numbers option developers has a way to decide for themselves whether they want to include them or not.

Fixes #321

[josevalim]

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