opt/yume-tagger: use upstream tag names if exist
opt/yume-tagger: use upstream tag names if exist

This commit is a solution to the following problem:

1. This party tag source reports that the given post should be tagged
   with "Welcome_to_the_NHK!"
2. Upstream post is already tagged with "NHK_ni_Youkoso!"
3. Upstream tag "NHK_ni_Youkoso!" actually has an alias,
   "Welcome_to_the_NHK!"
4. Autotagger wrongly thinks that "Welcome_to_the_NHK!" is a new tag

This commit makes autotagger realize when the given post already uses an
alias of the proposed tag.

rr-/dotfiles,rr-/dotfiles,rr-/dotfiles