Fixed color issues when viewing diffs in Vim

The default settings for diffs look fine when using Tomorrow. But if you use another theme like Tomorrow-Night, then it looks hideous:

![default](https://cloud.githubusercontent.com/assets/343450/3040936/f8c60d3c-e0ef-11e3-9823-cff31152f648.png)

This pull request fixes the look in each of the alternative schemes.

## Tomorrow-Night

![tomorrow-night](https://cloud.githubusercontent.com/assets/343450/3040978/4733cad6-e0f0-11e3-89b2-8205c57cb52c.png)

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## Tomorrow-Night-Blue

![blue](https://cloud.githubusercontent.com/assets/343450/3040950/1cdb3a44-e0f0-11e3-9ba1-da3e319cf0da.png)

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# Tomorrow-Night-Bright

![bright](https://cloud.githubusercontent.com/assets/343450/3040966/2cdfea2a-e0f0-11e3-880d-88b7ced8ff20.png)

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# Tomorrow-Night-Eighties

![eighties](https://cloud.githubusercontent.com/assets/343450/3040973/3ad94e5a-e0f0-11e3-8acb-9506b16fc327.png)

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The default Tomorrow theme is left as is, because it looks fine with the default settings

![tomorrow](https://cloud.githubusercontent.com/assets/343450/3040990/6cbe445c-e0f0-11e3-91dd-e372f56a9750.png)