config: set a default filter for text/plain

Avoid the following issue when running aerc with the default
configuration:

  No filter configured for this mimetype ('text/plain')

Use a very basic sed command to replace the default plaintext filter.

Fixes: bb0f180140 ("config: do not hardcode sharedir")
Signed-off-by: Robin Jarry <robin@jarry.cc>

config/aerc.conf

@@ -222,6 +222,7 @@ reply-to-self=true
 # against (non-case-sensitive) and a comma, e.g. subject,text will match a
 # subject which contains "text". Use header,~regex to match against a regex.
 #subject,~^\[PATCH=colordiff
+text/plain=sed 's/^>\+.*/\x1b[2;36m&\x1b[0m/'
 #text/html=pandoc -f html -t plain
 #image/*=catimg -w $(tput cols) -
 

filters/plaintext

@@ -1,16 +0,0 @@
-# vim: set ft=awk :
-BEGIN {
-	dim = "\x1B[2m"
-	cyan = "\x1B[36m"
-	reset = "\x1B[0m"
-}
-{
-	# Strip carriage returns from line
-	gsub(/\r/, "", $0)
-
-	if ($0 ~ /^On .*, .* wrote:/ || $0 ~ /^>+/) {
-		print dim cyan $0 reset
-	} else {
-		print $0
-	}
-}