Selector doc improvements (#153)

* Improvements to selector documentation

- Move non-applicable pseudo classes to "Non-Applicable Psuedo Classes"
- Rework some paragraphs
- Add links to developer.mozilla.org selector references
- Start adding usage examples
- Removing spacing after badge icons

* Add more selector examples

* Update escape related info

* Add more examples

* More examples

* More selector docs improvements

* More examples and document enhancements

* Finish pseudo class examples

* Finish up adding examples for namespaces

* Fix some typos

docs/src/dictionary/en-custom.txt

@@ -30,6 +30,7 @@ accessor
 amongst
 boolean
 builtin
+centric
 combinator
 combinators
 deprecations

docs/src/markdown/_snippets/selector_styles.txt

@@ -6,7 +6,6 @@ span.star::after {
   position: relative;
   display: inline-block;
   font: normal normal normal 16px/1 FontAwesome;
-  padding-right: .50rem;
   -moz-osx-font-smoothing: initial;
   -webkit-font-smoothing: initial;
   font-weight: 400;

docs/src/markdown/differences.md

@@ -18,13 +18,20 @@ about a malformed attribute, you may need to quote the value.
 For instance, if you previously used a selector like this:
 
 ```py3
-soup.select('[div={}]')
+soup.select('[attr={}]')
 ```
 
 You would need to quote the value as `{}` is not a valid CSS identifier, so it must be quoted:
 
 ```py3
-soup.select('[div="{}"]')
+soup.select('[attr="{}"]')
+```
+
+You can also use the [escape](./api.md#soupsieveescape) function to escape dynamic content:
+
+```py3
+import soupsieve
+soup.select('[attr=%s]' % soupsieve.escape('{}'))
 ```
 
 ## CSS Identifiers