Use less invasive listchars settings

[larissa]

I noticed that indentguides was changing my listchar settings and there was no option to turn that off. Looking at the code, it doesn't seem like that was even necessary, so I'm proposing these 2 changes:

• Put indentguides' listchar_guides in the beginning of the listchars list, that way it doesn't override any user set listchars.
• Remove trail from listchar_guides since that's not being used (as far as I can tell) by this plugin.

This renders the indentguides_tabchar unnecessary since the user can set whatever character they want directly on their vimrc listchars, but I left it there for compatibility.