Add some new stuffs

[ichirou2910]

In this PR I added a few new configurable stuff. Here is the list:

• show relative path of current file buffer (in case someone like me don't like it being in the statusline)
• make number and indicators customizable (what to display, color)
• readonly indicator as from ap#21

I also updated the docs, but somehow the indentation is really bad. :disappointed:

Please tell if I messed up anything. I revised it again and again but still I could have overlooked sth.