Added encode url & img size convert

[M97Chahboun]

I detected two issues :

1. convert img size
2. issue when have spaces on url

in this PR I try to fix this issues.
Thank you.

[pavelhoral]

Does this actually solve any issue with the library? AFAIK image size is not part of Common Mark specification and the library is definitelly not a place to fix incorrectly encoded URLs.

I am sure that if you need those custom features they can be achieved by custom rules https://github.com/mixmark-io/turndown#extending-with-rules.

[M97Chahboun]

OK, Thank you.