Add support for vim-airline again

[sheharyarn]

After the changes in #21 (specifically edfed09), support for vim-airline was removed with the message:

the current version is broken and I'm not using vim-airline at the moment

There's another closed issue (#26) that also brings this up, so I sent a PR (#30) to add it back but it's still broken. From the discussion in #30 (comment), I'm closing the PR and opening this issue to track this feature.

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For now, I'm sticking to the old version to keep using it with vim-airline:

Plug 'kaicataldo/material.vim', {'commit': '5aabe47'}