Make auto-generated file name configurable

[yegor-n-a]

1. Changed auto-generated timestamp as a UTC datetime
2. Added "file" option to configuration file:
   "file": { "SegmentSeparator": "_", "PrefixFormat": "yyyyMMddHHmmss" }
3. Added "Settings" and "File" classes as an infrastructural to generalize handling of configuration options
4. Updated documentation and NuGet package description respectively

[piotrekh]

Hi, sorry for the delay but I've been quite busy recently. I'll take a look at your PR when I find some spare time to sit down to it

[yegor-n-a]

Hi, sorry for the delay but I've been quite busy recently. I'll take a look at your PR when I find some spare time to sit down to it

No worries. It is completely understandable. Please, let me know when you finish. Thanks

[yegor-n-a]

@piotrekh Also, I am about to complete implementation of support for sqlscriptoptions which were introduced in DbUp 4.2. Please, see the details here: https://github.com/DbUp/DbUp/blob/master/docs/more-info/run-group-order.md Therefore, please, provide your feedback as soon as you can in order to simplify merging and pushing the aforementioned improvements. Thank you.

[yegor-n-a]

@piotrekh Hi Piotr. I have just added new parameter -ExecutionMode to support building of ScriptOptions and improve integration of script types available for DbUp 4.2 and higher. Please, review when you have a chance. Thank you.