Preserve whitespace characters

Related to #37 :

A space is 0x20:
```
% echo -n ' ' | hexdump                                   
0000000 0020                                   
0000001
```
Here, I'd expect to see 3x `20`, but I only see it once:
```
% echo -n ":sparkles:   Three spaces" | emojify | hexdump 
0000000 9ce2 20a8 5420 7268 6565 7320 6170 6563
0000010 0a73                                   
0000012
```