Change theme.ini format to support Python ConfigParser defaults

I think the theme.ini file should be able to be loaded with ConfigParser defaults. It currently cannot.

Expected

1. Default ConfigParser will load theme.ini without exception
2. Values will not have quotes around them
3. PyCharm will not show errors in the ui due to keys starting with spaces

Actual

1. theme.ini raises configparser.MissingSectionHeaderError: File contains no section headers. because the contents do not start with a section header. It starts with keys.
2. Because values have quotes around them, when printing the values, the quotes remain by default
3. PyCharm highlights in red, name under the author section. Removing the leading spaces removes this error/warning.

Steps to repro

code

from configparser import ConfigParser
parser = ConfigParser()
with open("theme.ini", 'r', encoding='utf-8') as f:
    parser.readfp(f)
print(parser)

sample config

name = "ThemeName"
license = "MIT"
licenselink = "https://google.com"
description = "Simple Theme "
homepage = "https://github.com"
tags = ["simple", "minimal", "blog"]
features = ["blog"]
lektor_required_version = 3.1

[author]
    name = "foobar"
    homepage = "http://github.io/"

[original]
    author = "origAuthor"
    homepage = "https://github.com"
    repo = "https://github.com/repo"

[packages]
    lektor-disqus-comments = 0.2

Suggestion

1. Require all keys be in sections
2. Do not quote values
3. Do not start keys with leading spaces
4. EDIT: Lists/Arrays should just be a CSV string

Sample Suggested theme.ini

[theme]
name = ThemeName
license = MIT
licenselink = https://google.com
description = Simple Theme
homepage = https://github.com
tags = simple, minimal, blog
features = blog,
lektor_required_version = 3.1

[author]
name = foobar
homepage = http://github.io/

[original]
author = origAuthor
homepage = https://github.com
repo = https://github.com/repo

[packages]
lektor-disqus-comments = 0.2

[nixjdm]

There is currently nothing public that parses this file yet, but @Andrew-Shay is working on a private plugin for lektor-website that would use it, and of course the other theme tickets would start using it.

I want to stay consistent with what we’re doing elsewhere in this project. We don’t currently (and likely won’t for a while if ever) support alternatives to inis elsewhere in lektor, and they shouldn’t be supported here separately.

If you look through the docs, you’ll see the de facto standard in lektor projects is to use csv strings instead of arrays. Good catch. So that’s the way we should do it in these files too.

[nixjdm]

This is completed as far as this repo is concerned in #601, and nearly complete everywhere. I’ve done all I can. I pulled your PRs @Andrew-Shay, updated the lektor-themes readme, and my own theme, and submitted a few quick PRs to the other themes, owned by @Andrew-Shay, @rlaverde, and @hannylicious.