language and langid fields are missing when doing bib -> yml conversion

[Suzukeh]

I found that language and langid are missing when doing BibTeX(.bib) to Hayagriva YAML(.yml) conversion using Hayagriva CLI.

Input(.bib):

@book{test_enUS,
  langid   = {american},
  language = {en-US}
}

Expected output(.yml):

test_enUS:
  type: book
  langid: american
  language: en-US

Real output:

test_enUS:
  type: book

I ran hayagriva test.bib > test.yml using Hayagriva CLI 0.8.0.
These formats of fields in test.bib are based on an output of Better BibTeX for Zotero.

I think Hayagriva has both fields because of #126, but I couldn't find the langid field in file-format.md. If langid is not supported, you don't have to add it.