add new method configFileChain

[satotake]

Although MergeInConfig has quite important functionality, it is necessary to repeat SetConfigFile in order to apply to multiple config files.
This PR add to the wrapper function MergeConfigFileChain of MergeInConfig to make it cheaper to use.

• set slice of config files as v.configFileChain via SetConfigFileChain
• index of slice v.confiFileChain define the priority for merge config
  • larger number of index means higher priority (namely, merged later)

Actually, I would like to use this method for hugo considering gohugoio/hugo#2735

[CLAassistant]

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