Add support for @bem/cell

[Yeti-or]

closing #10
Example of new API:

var scheme = require('bem-fs-scheme');
scheme().path(
    new BemCell({
        entity: new BemEntityName({
            block: 'a',
            elem: 'e1',
            mod: {name: 'mn', val: 'mv'}
        }),
        tech: 'js',
        layer: 'common.blocks'
    })
)
// "common.blocks/a/__e1/_mn/a__e1_mn_mv.js"

options for bem-naming could be passed as second argument:

var scheme = require('bem-fs-scheme');
scheme().path(bemCell, {naming: {elem: '%%%', mod: '###'}})

[Yeti-or]

If you noticed I didn't break API cause this package is already 1.0.0.
Another options that I see:

1. 2.0.0
2. Add different method for bem-cell
3. Create new module:

var path = require('@bem/path')('nested');
path.resolve(new BemCell({
        entity: new BemEntityName({
            block: 'a',
            elem: 'e1'
        }),
        tech: 'js',
        layer: 'common.blocks'
    })
);

[Yeti-or]

or even:

var path = require('@bem/path');
path.resolve(new BemCell({
        entity: new BemEntityName({
            block: 'a',
            elem: 'e1'
        }),
        tech: 'js'
    })
);

path.flat().resolve(new BemCell({
        entity: new BemEntityName({
            block: 'a',
            elem: 'e1'
        }),
        tech: 'js'
    })
);

[Yeti-or]

@zxqfox @blond @skad0 take a look.

[skad0]

Maybe we should throw an exception if there isn't BemCell?

[Yeti-or]

@skad0 add assertion

[skad0]

lgtm