Include Roslynator.Analyzers and Roslynator.CodeFixes with more strict default severity levels

Introduce https://github.com/JosefPihrt/Roslynator, which has 500+ analyzers, and also a large number of refactorings.

Lists of each:

http://pihrt.net/Roslynator/Analyzers
http://pihrt.net/Roslynator/Refactorings

Here's a very preliminary set of some recommendations for default severities. This is by no means exhaustive and is only the set that I came up with upon looking at the Info level severity issues from a single project I'm working on at the moment.

For some of these that aren't already covered, corresponding rules should be added to the C# standards.

``` editorconfig
# RCS1001: Add braces (when expression spans over multiple lines).
dotnet_diagnostic.RCS1001.severity = error

# RCS1003: Add braces to if-else (when expression spans over multiple lines).
dotnet_diagnostic.RCS1003.severity = error

# RCS1007: Add braces
dotnet_diagnostic.RCS1007.severity = error

# RCS1044: Remove original exception from throw statement
dotnet_diagnostic.RCS1044.severity = error

# RCS1123: Add parentheses according to operator precedence.
dotnet_diagnostic.RCS1123.severity = error

# RCS1037: Remove trailing white-space.
dotnet_diagnostic.RCS1037.severity = none

# RCS1036: Remove redundant empty line.
dotnet_diagnostic.RCS1036.severity = none

# RCS1170: Use read-only auto-implemented property.
dotnet_diagnostic.RCS1170.severity = warning

# RCS1213: Remove unused member declaration.
dotnet_diagnostic.RCS1213.severity = warning
```