bug#29108: 25.3; ERC SASL support

[Alex Branham]

Since freenode requires SASL support if you're connecting from certain 
networks, it would be nice if ERC supported connecting via SASL. There seems to 
have been some work on this a few years ago: https://github.com/jane-lx/erc-sasl

Thanks!

In GNU Emacs 25.3.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.19)
 of 2017-09-16 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.11905000
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix