refs #20 Added a feature generating a choice from file names in a directory.
refs #20 added comments and help documents.
refs #20 wrote input validations.
closed #20 wrote tests for FilenameChoiceListProvider.
Merge remote-tracking branch 'remotes/ikedam/feature/20_FilenameChoide'

Conflicts:
	src/main/resources/jp/ikedam/jenkins/plugins/extensible_choice_parameter/Messages.properties
	src/main/resources/jp/ikedam/jenkins/plugins/extensible_choice_parameter/Messages_ja.properties
Updated documents for File Choice Parameter.