Specify backend parameters with prefixes in projects.cfg

[osma]

Current situation:

[fasttext-fi]
name=fastText Finnish
language=fi
backends=fasttext
analyzer=snowball(finnish)
dim=500
lr=0.25
epoch=30
loss=hs
limit=100
chunksize=24

Desired situation:

[fasttext-fi]
name=fastText Finnish
language=fi
backends=fasttext
analyzer=snowball(finnish)
fasttext.dim=500
fasttext.lr=0.25
fasttext.epoch=30
fasttext.loss=hs
fasttext.limit=100
fasttext.chunksize=24

By using the backend name as a prefix (similar to how --backend-param works on the CLI), we can support (with backend aliasing, which will become a separate issue) multiple backends of the same type for the same project. Also it makes it clear whether parameters are project- or backend-specific.

[osma]

This may become unnecessary after #218

[osma]

Not necessary anymore due to #218.