syntax: email: rename file and syntax, away from the mistaken 'mutt'

Also, colorize lines that are multiply quoted, and email addresses,
and URLs.

syntax/Makefile.am

@@ -8,6 +8,7 @@ pkgdata_DATA =	asm.nanorc \
 		debian.nanorc \
 		default.nanorc \
 		elisp.nanorc \
+		email.nanorc \
 		fortran.nanorc \
 		gentoo.nanorc \
 		go.nanorc \
@@ -21,7 +22,6 @@ pkgdata_DATA =	asm.nanorc \
 		makefile.nanorc \
 		man.nanorc \
 		mgp.nanorc \
-		mutt.nanorc \
 		nanohelp.nanorc \
 		nanorc.nanorc \
 		nftables.nanorc \

syntax/email.nanorc

@@ -0,0 +1,17 @@
+## Syntax highlighting for emails.
+
+syntax email "\.eml$"
+
+# Quoted lines.
+color green "^>.*"
+color cyan "^> ?>.*"
+color yellow "^> ?> ?>.*"
+
+# Email addresses.
+color brightmagenta "<[^@]+@[^@]+>"
+
+# URLs.
+color brightblue "(https?|ftp)://\S+\.\S+[^\s.)]"
+
+# Signatures, even quoted ones.
+color yellow start="^>* ?-- $" end="^>* ?$"

syntax/mutt.nanorc

@@ -1,9 +0,0 @@
-## Here is an example for quoted emails (under e.g. mutt).
-
-syntax mutt
-
-# Quoted lines.
-color green "^>.*"
-
-# Signatures, even quoted ones.
-color yellow start="^>* ?-- $" end="^>* ?$"