Add support for Plaintext

https://www.conwaylife.com/wiki/Plaintext

For example, Glider:

```
!Name: 28P7.2
!Author: Dean Hickerson
!A period 7 oscillator that is tied with burloaferimeter and 28P7.1
!as the smallest.
!www.conwaylife.com/wiki/index.php?title=28P7.2
.OO
..O
.O
.OO.......OO
...O....O..O
.OO..OO..OO
O..O....O
OO.......OO
..........O
.........O
.........OO
```