Conditional classes added

two syntaxes are available – string and object

const b = bem('tile');

const isActive = false;
const isDark = true;
assert.equal(b(`--active?${isActive}.--dark?${isDark}`), 'tile tile--dark');

const b = bem('tile');

const isActive = true;
const isDark = false;
assert.equal(b({
  '--active': isActive,
  '--dark': isDark
}), 'tile tile--dark')