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.

ftplugin/markdown.vim

@@ -0,0 +1 @@
+let g:table_mode_corner = '|'

ftplugin/rst.vim

@@ -0,0 +1,2 @@
+let b:table_mode_corner_corner = '+'
+let b:table_mode_header_fillchar = '='