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Add `!default` flag to vars.

Fixes #10
Ensure that silent push and pull classes work given that they cannot
make use of the push and pull attribute selector.

Fixes #9
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DRY out relative positioning used on silent push and pull
Add grid centring class
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Ensure centred grids properly extend regular ones
Add licensing

Closes #11
Restore proper text alignment in grid items that use centred grids
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Allow users to opt out of the markup-fix for whitespace. Thanks to
@kevva
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Update documentation
Merge branch 'dev'