Allow BooleanSqlToggle to share a single <connnectionStrings> named connection

[jason-roberts]

See #88 for more background

<connectionStrings>
    <add name="primary" connectionString="connection-string" />
</connectionStrings>

<appSettings>
    <add key="FeatureToggle.MyFeatureToggle.ConnectionStringName" value="primary" />
</appSettings>

[craig-wagner]

Just submitted a pull request (#97) for this feature (with unit tests). This is the first time I've tried contributing. Followed the instructions on https://guides.github.com/activities/forking. Let me know if anything needs improvement.

[jason-roberts]

@craig-wagner - do you want to take a quick look at this d6a1564 just to check it's still operating as you expected? If all is well we can get v3.2 released this week :)

[craig-wagner]

@jason-roberts - Looks like it still works as I intended.