Allow specifying a config file

[Oondanomala]

Allows setting a custom ExifTool config file.
Because the -config option has to be the first one specified, it required modifying the strategies directly.

The optimal solution would have been to add configPath as a parameter for execute() just like whats done for the ExifTool path, but that would have broken backwards compatibility, so instead I added a setConfigFilePath() method to ExecutionStrategy.

Closes #247

[mjeanroy]

Why the config option cannot be added to the StandardOptions?
Then, we would "just" need to handle it first in the serialize method?

[Oondanomala]

Because the strategies always add -sep before the given arguments and -config must be the first option.
And it cant add that after since then it would be after -execute.
I guess each strategy could manually insert -sep in between the arguments, but that would be worse IMO.

[Oondanomala]

It might be better to make ExifTool.toArguments add -sep instead,
BUT to keep backwards compatibility getRawExifToolOutput would have to be modified to add -sep to the argument list,
which would in turn break compatibility for programs that subclass ExecutionStrategy, make it not add -sep, and use getRawExifToolOutput, expecting it to not have -sep as an argument.
Also it would be a compatibility breakage for every ExecutionStrategy subclass since they would now get an unexpected -sep argument, but that might not be as big of a deal as AFAIK duplicate -sep arguments get ignored.

Honestly the whole api is very inconsistent (like #205), but sadly it cant be cleaned up without a compatibility breakage.