settings: Bot assumes + for modes

[dgw]

The core.modes setting is assumed to be just a string of letters, and the leading + is assumed by coretasks:

sopel/sopel/coretasks.py

Lines 155 to 156 in a33caf1

	modes = bot.config.core.modes
	bot.write(('MODE', '%s +%s' % (bot.nick, modes)))

@cottongin rightly pointed out on IRC that sometimes it's desirable to remove modes the IRC server sets by default. While some IRCds will happily accept MODE nickname +-abc (like freenode's), it's not a universal workaround.

I'm happy to add this to 7.1 because 1) it's a pretty trivial change to implement and 2) there's an obvious backward-compatible way to parse the setting.

Proposal: Add the leading + automatically only if the core.modes setting doesn't contain a prefix character (+ or -).