Make custom formatting in Table work properly with ANSI colors

[astrofrog]

If using ANSI colors in a table formatting function, the columns are not padded correctly due to the fact that the ANSI color commands are included in the length of the string:

In [1]: from termcolor import colored

In [2]: def fmt(x):
   ...:     if x > 0:
   ...:         return colored(str(x), 'green')
   ...:     else:
   ...:         return colored(str(x), 'red')
   ...:     

In [4]: from astropy.table import Table
t =
In [5]: t = Table()

In [6]: t['a'] = [-1, 1, 0, 3, -3]

In [7]: t['b'] = [-1, 1, 0, 3, -3]

In [8]: t
Out[8]: 
<Table length=5>
  a     b  
int64 int64
----- -----
   -1    -1
    1     1
    0     0
    3     3
   -3    -3

In [9]: t['a'].format = fmt

In [10]: t['b'].format = fmt

In [11]: t
Out[11]: 
<Table length=5>
     a           b     
   int64       int64   
----------- -----------
-1 -1
 1  1
 0  0
 3  3
-3 -3

The color works correctly though:

It would be nice if we could do the right thing™️ here though and auto-strip the ANSI color commands from strings when figuring out the width of the columns. Feel free to close this if it would be much too difficult though.

[taldcroft]

Is there any kind of fast library that can strip out ANSI color commands or render ANSI text as plain ASCII in order to compute length? I think that without a pre-existing library to do the job (fast) this would be a bit of work and would impact performance.