EHN: to_{html, string} col_space col specific
- [ ] closes #28917
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

In to_html, to_latex and to_string, ```col_space``` arg now accepts a list or dict where we can mention the column name to change only a specific column's width.
Haven't add test for to_latex and to_string since they too don't have test for the old col_space and I cannot think of anything less cumbersome than to count added whitespace.
@quangngd you still interested in working on this? > @quangngd you still interested in working on this?

Yes, Im still waiting for feedbacks Hello @quangngd! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:











There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-06-18 06:38:51 UTC Tests added for to_string.
Decided to drop to_latex due to lack of latex experience.
FYI, the current implementation for col_space in to_latex doesn't work either since it just simply add spaces between strings (like in to_string). @quangngd - thanks for the PR mind fixing up the test failures?  @alimcmaster1 done cc @WillAyd @simonjayhawkins for a look Great PR - thanks a lot @quangngd !