Support the "Info" category for flake8

[remcohaszing]

Environment data

• VS Code version: 1.22.2
• Extension version: 2018.3.1
• OS and version: Ubuntu 17.10
• Python version: 3.6
• Type of virtual environment used: N/A
• Relevant/affected Python packages and their versions: flake8==3.5.0 flake8-import-order==0.17.1

Actual behavior

Flake8 plugin configuration settings don’t match the settings schema in vscode, even though the settings do work.

Expected behavior

Flake8 plugin settings are not marked as unknown in the vscode settings file.

Steps to reproduce:

1. Install flake8

2. Install flake8-import-order

3. Open a file with the following contents:

   from foo import bar
   
   import sys

4. Notice how this triggers an info level problem

5. Add this line to your vscode settings:

     "python.linting.flake8CategorySeverity.I": "Warning",

6. Notice this gives a warning in the settings file

7. Save or reopen the Python file

8. Notice that the severity level has changed from info to warning

Logs

N/A

[brettcannon]

We should come up with a flexible mechanism for classifying flake8 issues/categories so that we can cover this need as well as any other future categories a plug-in might add.

Idea I head was a dict whose keys get put into a trie which we can the use to find the specified category or issue's classification.