Add support for double-width characters

[ulidtko]

Fix SublimeText/ElasticTabstops#14

[ulidtko]

I hope it's obvious from the code, but what I did is:

• defined column_width function which takes into account wide characters;
• cell_widths_for_row uses the above instead of plain len to properly calculate cell widths;
• adjust_row for the most part works as before, except that difference is re-synchronized inside the loop by column widths instead of character offsets. Unfortunately, this'll work slightly slower than before — but I didn't encounter any noticeable slowdown on my 10kiB TSV file with 10 columns.

And no changes beyond that, pretty simple in fact.

[adzenith]

Sorry for the huge delay! Looks great. I had a couple quick comments. Thanks for the PR!