Support indented PO files
Some of polib's checks to determine the context of a token is based on hard-coded strings like 'msgid "'.
Unfortunately, those checks cannot take into account indented PO files (-i option in gettext) where there may be more than a single space between "msgid" and the opening quote.
This is true for most tokens (msgctxt, msgid, msgstr, msgid_plural). I'm not sure about special comments like "#| msgid", though I'd suggest supporting multiple spaces there too, to make polib more lax about its inputs.