Center center-aligned cells

[nobu]

Center center-aligned cells.

Description

Center-aligned cells were formatted as left-aligned.

When markdown-table-align at the following table:

| A | B |
|:---:|:--:|
| a | bbbbbb |
| aaa | bbbb |

Before:

| A   | B      |
|:---:|:------:|
| a   | bbbbbb |
| aaa | bbbb   |

After:

|  A  |   B    |
|:---:|:------:|
|  a  | bbbbbb |
| aaa |  bbbb  |

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
• All new and existing tests passed (using make test).

[syohex]

Thanks.