Add support for lgr logging package

[aryoda]

lgr: a logging package for R built on the back of R6 classes. Supports logging of data in objects.

Related to master feature request #42 (support more major logging frameworks)

[aryoda]

Important design decisions to be taken:

1. Which supported logging framework shall be used if multiple are installed?
2. How to specify a default logging framework to choose another one than the one picked by tryCatchLog at start-up?
3. How to switch the logging framework once tryCatchLog was loaded and picked the "wrong" logging framework?

I suggest this behaviour:

1. The supported logging frameworks shall have a predefined fixed search-order to have a deterministic behaviour.
   Therefore tryCatchLog shall also choose the first supported installed package (not loaded package!)
   Newly supported logging frameworks will be added at the end of this list.
2. A new option will be introduced to specify the preferred logging framework. If it is not installed the standard behaviour of 1. is used to pick a logging framework.
3. A new function "enableXYLoggingFramework" shall be introduced (either a generic function with the package name as argument or one explicit function per supported logging framework (TODO: Decide this)

[aryoda]

Damn, I forgot to implement a unit test for set.logging.package() and have found a bug during testing of another issue:

options("tryCatchLog.preferred.logging.package" = "tryCatchLog")
set.logging.package("futile.logger")
> Error in match.arg(logging.package.name, several.ok = TRUE) : 
> 'arg' should be one of “tryCatchLog”

options("tryCatchLog.preferred.logging.package" = NULL)
> set.logging.package("futile.logger")
[1] "futile.logger"

The reason is that match.arg uses the list of allowed values from the option here (since it is set) and does not allow other values anymore :-(

I will fix this soon and add a unit test too...

[aryoda]

Bug fixed in this commit: f8a88fa