Support displaying shortcodes instead of Emojis in messages (#222)

docs/example_config.json

@@ -10,6 +10,7 @@
 	},
 	"settings": {
 		"log_level":                  "warn",
+		"message_shortcode_display":  false,
 		"reaction_display":           true,
 		"reaction_shortcode_display": false,
 		"read_receipt_send":          true,

docs/iamb.5.md

@@ -53,6 +53,10 @@ overridden as described in *PROFILES*.
 > Specifies the lowest log level that should be shown. Possible values
 > are: _trace_, _debug_, _info_, _warn_, and _error_.
 
+**message_shortcode_display** (type: boolean)
+> Defines whether or not emoji characters in messages should be replaced by
+> their respective shortcodes.
+
 **reaction_display** (type: boolean)
 > Defines whether or not reactions should be shown.