setting Array as `custom` replaces numbers

[deyhle]

I found a bug when using the option to set an Array of allowed characters by giving an Array to the custom option: it replaces numbers in the string with the allowed characters.

Example:

getSlug('0.1,', { custom: ['.', ','] })
// expected: '0.1,' (no replacement)
// actual: '0011' (. is replaced with 0, , is replaced with 1)

The reason is that the check if the custom property is an object also returns true if it is an Array. Because of that a replacement map is set for the Array indizes and 0, 1, … are replaced with the custom allowed characters.

This pull requests adds a test and a fix for that by additionally checking if the object is not an Array. It would be great if you could publish an update to npm with that fix

[deyhle]

Could you please merge this pull request and publish to npm?