Read the existing MANIFEST.in file for files to ignore.
Add list for ignoring regular expressions.

Use this to support ignoring patterns that are in MANIFEST.in with the 'exclude' keyword.
Test strip_sdist_extras with a MANIFEST.in file.
changelog
Use the 'with' statement for opening files.

This avoids ResourceWarnings on Python 3.x and leaking file descriptors on PyPy.
Remove reduntant 'strip'.

'split' already ignores leading and trailing whitespace.
Do not make a dot into a magical wildcard character.
Keep the indentation visually clear in the test.

Remove leading whitespace programmatically with textwrap.dedent.
strip not needed in combination with split.
Better whitespace checking when parsing a MANIFEST.in line.

It seems tabs are accepted in the manifest.
We now split each line to check for a command (exclude, prune, etc) and arguments.
Make sure prune gets no path separator.
Use textwrap.dedent again to keep the code clear.
Warn when we cannot parse recursive-exclude.
Fix recursive-exclude when pattern does not have wildcards.
Fix recursive-exclude for pattern not starting with wildcard character.
Use the warnings module to print warnings.

Ignore warnings in one known spot in the tests.
This keeps the test output clean.
Merge pull request #19 from mauritsvanrees/maurits-read-manifest

Read the existing MANIFEST.in file for files to ignore.