Handle plurals with count tag of blocktrans

[amorphobia]

Currently, several plurals are generated with pluralize filter. I found it wired when translating to Chinese.

Usually, nouns do not have plural forms in Chinese, and the %(plural)s term will append an extra "s". However, if I delete %(plural)s in the msgstr, it would not pass the checker and leave original English message there.

Instead, if count tag is used, it can be solved by only translate msgstr[0]. I made a commit in my forked repo. But this may break all other translations, so I did not open new PR. If you think it make sense, I will open one.

[cdubz]

Interesting -- I didn't know about the count tag. This does look like a better solution and I'd take PR for it. Breaking other translations is unfortunate but I think it's worth using this as a better approach anyway.

[cdubz]

#473 is merged. Thanks for contributing!