pastebuf: fix sgr attribute parsing

weechat reset previous attribute by default everytime weechat.color is
called unless the value is prefixed with a pipe.

also added more supported sites and bind alt+c to close the buffer.
pastebuf: command inside buffer & use temp file

- it is possible to input command inside paste buffer now. currently
  there's only 1 command (`lang`) to change the syntax language of
  current buffer.
- move temporary storage from buffer's localvar to a temp file
- added option show_line_number
- added option indent_width
- expand tabs at the start of line to spaces
- arrow up/down now will always scroll 1 line. ctrl+up/down will scroll
  10 lines. ctrl+left/right will scroll 10 chars.
pastebuf: support multiple file in a gist

this requires lua cjson module. if the module doesn't exist, only the
first file will be fetch. oh, and the gist will be highlighted
automatically ;)