Add plain text support

[Kajiih]

Discussed here: #1578
I chose white chip because there is no color in linguist.

ASCII art looks like this in my terminal:

I found that white was a bit too aggressive, so I picked a darker shade.

I also tried this, but it's pretty small:

Feel free to edit anything!

[o2sh]

Since there’s no logo for this, we can be creative. Here’s a suggestion for ASCII art that suggests “plain text”:

  ascii: |
    {0} -----------------
    {0}| ! " # $ % & ' ( |
    {0}| ) * + , - . / 0 |
    {0}| 1 2 3 4 5 6 7 8 |
    {0}| 9 : ; < = > ? @ |
    {0}| A B C D E F G H |
    {0}| I J K L M N O P |
    {0}| Q R S T U V W X |
    {0}| Y Z [ \ ] ^ _ ` |
    {0}| a b c d e f g h |
    {0}| i j k l m n o p |
    {0}| q r s t u v w x |
    {0}| y z { | } ~     |
    {0} -----------------
  colors:
    ansi:
      - white

[Kajiih]

I think it does a good job at suggesting plain text!

[o2sh]

maybe something less random :

  ascii: |
    {0} -----------------
    {0}| .txt ========== |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0}|                 |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0} -----------------
  colors:
    ansi:
      - white

[Kajiih]

I experimented with some variations of this, I think my favorite is something like:

  ascii: |
    {0} -----------------
    {0}| .txt ========== |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0}| ============    |
    {0}|                 |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0}| =============== |
    {0} -----------------
  colors:
    ansi:
      - white

[o2sh]

Ok, you can commit it and I'll approve the PR