Added support for customising date in filename

[RandomTIP]

Address (Partially): #243

Changes:

• PathRoller now recognizes and uses {Date} placeholder in file name.
• Regex matching and directory search patterns have been adjusted for use with {Date} placeholder

This update should Enable users to use following configuration:

var config = new LoggerConfiguration()
            .WriteTo.File("{Date}-log.txt", rollingInterval: RollingInterval.Day);
var log = config.CreateLogger();

Unit Tests Added and Updated in:

• TemplatePathRollerTests
• RollingFileSinkTests

[RandomTIP]

I know this PR came out of nowhere, but I needed support for this for my project and thought I would send a PR to the original repo as well since the changes are quite simple and straightforward.

This partially addresses #243 without any sort of high level customization, I wanted to make it as simple as possible because, after one or two days worth of searching, I think most of the requests were only related to Date customization in filename.

I would appreciate and take into considaration any and all notes on this one

[nblumhardt]

Thanks @RandomTIP. This functionality is something we specifically decided to leave behind in Serilog.Sinks.RollingFile - using that package is the way to go, if you need {Date}, but otherwise, using your own fork of this sink is going to be the way forwards. Thanks again!

[RandomTIP]

Thanks for the information. I was not able to gather that much myself for some reason... I assumed Serilog.Sinks.RollingFile was deprecated and that is why I overlooked it.