Add ansi_to_text method

[fetus-hina]

It remove escape sequences and returns plain text.
For example,

var input = "\033[31mred\033[0m";
console.log(ansi_up.ansi_to_html(input)); // '<span style="color:rgb(187, 0, 0)">red</span>'
console.log(ansi_up.ansi_to_text(input)); // 'red'

Users can get safely plain text from escape sequence marked up string.

[drudru]

Really good stuff - I'll push the npm on Monday - thanks!