Updated UpdateLineBorder function

Rather than get a variable number of arguments to make it optional I now
require one argument, the line number to operate on for the function
UpdateLineBorder. Makes sense to be this way.

Also improved some conditionals for better readability.
Fixed Tableize indentation issue

When applying Tableize on an indented code, the table was created
however the indentation was lost in the process. Earlier the function
s:ConvertDelimiterToSeparator(line) was replace the ^ (start of line),
the g:table_mode_delimiter and $ (end of line) with
g:table_mode_separator and then processing each line for creation of the
table around it, hence since the start of line was replaced it was
causing the indentation to be lost. Fixed this by replacing ^\s*\zs\ze.
(matches just before the first non-blank character) along with
g:table_mode_delimiter and $ and hence the indentation is preserved.