Automatically setup table options

Readme mentions settings for Markdown and ReST, but we can apply them
automatically for the current filetype. If a user is editing a no
filetype scratch buffer, they'll still need to configure these options
themselves. But this provides better defaults when editing a buffer of
these filetypes.

Adds get_buffer_or_global_option that could be used for any other
buffer-local (filetype-specific) options.

README.md

@@ -113,6 +113,7 @@ To get ReST-compatible tables use
     | Sherlock Holmes | 221B Baker Street        | 0987654321 |
     +-----------------+--------------------------+------------+
 
+Markdown and ReST filetypes have automatically configured corners.
 
    You can also define in a table header border how it's content should be
    aligned, whether center, right or left by using a `:` character defined by