Get enough precision for calculating the width of the movie.

The solution is to use more `M` characters to get a more accurate
measurement of its width. Currently 200 `M` characters are used in
stead of 10.

See: asciinema/asciinema.org#169
Merge pull request #2 from fd/fd-get-enough-precision-for-width-calculation

Get enough precision for calculating the width of the movie.