Language detection method in CLI

[osma]

Issue #631 proposed adding a language detection method in the REST API and CLI. The REST API part was implemented in PR #659, closing the issue. But the CLI part still remains to be done. This issue attempts to specify the CLI part.

Here is an example how it could work:

$ echo "this is some example text in an unknown language" | annif detect-language --languages fi,sv,en
en      1.0000
fi      0.2000
sv      0.2000
-       0.0000

Some notes:

• the new command is annif detect-language
• the candidate languages are given as the --languages parameter, separated by comma
• the output is TSV (language code and score)
• the results are ordered by decreasing score (best guess first)
• - means unknown language (same as null value in the REST API)
• the number of candidate languages is not limited (unlike the REST API method where the limit is 5 due to memory usage concerns)