Math operator fixes

[kristerkari]

• Don’t format math operators inside function calls
• Don’t format math operators at the beginning of a string, but add spaces before a number: /[+\-*/%](?=\$|\()/g => /(?!^)[+\-*/%](?=\$|\(|\d)/g

With this input:

$height: em(240/2);
$easing: cubic-bezier(.35, 0, .765, -0.120);

div {
  margin-top: -($height/2);
}

CSSfmt output before:

$height: em(240 / 2);
$easing: cubic-bezier(.35, 0, .765, - 0.120);

div {
  margin-top: - ($height/2);
}

and after fixes:

$height: em(240/2);
$easing: cubic-bezier(.35, 0, .765, -0.120);

div {
  margin-top: -($height / 2);
}

[matype]

@kristerkari Thank you for your contribution! Please fix this PR and #66 to merge :)

[kristerkari]

@morishitter done!

[matype]

@kristerkari Nice work!