fix(string-utils): detect [0,1] ranged values as numbers

[antongolub]

closes #389

This PR brings back detection regular decimal fractional numbers (0.1, 0.0313) as numbers.

before:

imagemin-cli %  ./cli.js --plugin.pngquant.quality={0.1,1} --plugin.foo.bar=0.2 foo.png
{
  pngquant: { quality: [ '0.1', 1 ] },
  foo: { bar: '0.2' }
}

after:

imagemin-cli %  ./cli.js --plugin.pngquant.quality={0.1,1} --plugin.foo.bar=0.2 foo
{
  pngquant: { quality: [ 0.1, 1 ] },
  foo: { bar: 0.2 }
}

[antongolub]

@bcoe, could you take a look?

[bcoe]

Looks good to me, but I would like to test against yargs' suite.

[bcoe]

@antongolub thank you for the contribution, works great with yargs.