table.align["xyz"] = "l" doesn't work when color is added ('\033[34mxyz\033[0m') to the header fields.

```
What steps will reproduce the problem?
1. Set header field to left table.align["xyz"] = "l"
2. Wrap header field with ansi colors table.header(['\033[34mxyz\033[0m'])
3. Print table

What is the expected output? What do you see instead?
All the values under field xyz should be aligned to the left, but they're
instead aligned at the center.

What version of the product are you using? On what operating system?
Python 2.7.2 OS X 10.8.4 prettytable (0.7.2)

Please provide any additional information below.
It would be a nice feature to be able to add color to fields and values.
```

Original issue reported on code.google.com by `SShagin...@gmail.com` on 21 Aug 2013 at 4:26

Reproduce the issue using the alignment and ANSI-colored header steps in the report, then trace the table formatting and width/alignment handling. Done means the xyz column remains left-aligned when its header contains color escape sequences, with the existing table output behavior preserved.

Written by the indexing model from the issue text.