Make formatting of README.md more consistent

* Replaced triple backticks with single backticks for inline code
* Formatted `parse()` and `?p` as code
* Replaced HTML bold tags with markdown asterisks
Heavily tweak README.md

* Fix grammar and spelling wherever it was wrong
* Tweak some sentences for style or clarity
* Add some additional details to the intro section and the placeholder descriptions
Add ?k and ?m placeholders for bulk inserts
Update README.md to detail new placeholders
Improve comments
* Add documentation and examples of ?m and ?k placeholders
* Fix typos
* Rewrite a couple of docstrings that were hard to understand or had bad English
Add examples from code comments to README.md
Tiny English fix
Improve ?k example to show array format
Fix bug in ?m example
Fix bug in ?k example
Merge branch 'master' of https://github.com/ExplodingCabbage/safemysql