More suggested edits for checklist

[mblue9]

Apologies for another PR so soon after the last one! but on having another look at the checklist I've got some more suggested edits. It's some changes to formatting and also some things that I thought I'd suggest including as someone new to wrapping (they're things that helped me that I thought might help others).

In this PR I've:

• reduced nesting of items to reduce random-looking bolding
• created a separate test section
• added more links
• added mentions and links to the things I'd never heard of til I started wrapping (Cheetah, CDATA, Rst, flake8, PEP 8)
• added the things needed to pass planemo lint
• added things flagged in IUC reviews (validators, filters, indentation etc)

I also have a couple of questions on things that came up while doing this that I was wondering if anyone might know?

• For links to more info in the checklist, I put links to the IUC Best Practices doc when that had info, otherwise I put links to the XML schema, but for Macros, while there's a link called "macros" on the IUC page (here: http://galaxy-iuc-standards.readthedocs.io/en/latest/best_practices/tool_xml.html#coding-style) that goes to the XML schema (here: https://docs.galaxyproject.org/en/master/dev/schema.html), there's no mention of Macros there that I can find. So I linked to the Macros info in Planemo (here: http://planemo.readthedocs.io/en/latest/writing_standalone.html#macros). Just wondering if Planemo is the best source for that Macros info?

• Is the info here on Packages still current or with conda is it considered deprecated now: http://galaxy-iuc-standards.readthedocs.io/en/latest/best_practices/package_xml.html

• The Best Practices for Data Managers section here: http://galaxy-iuc-standards.readthedocs.io/en/latest/best_practices/tool_xml.html#data-managers just has "TODO" Is there any info that could go there? Or where would be the best place to find that info?

[mblue9]

Thanks @nsoranzo! I've made those changes.