Added composer.json support

[vinkla]

This pull request adds Composer support to the autoloader. Couple of questions before this pull request can be merged:

1. Should it be named roots/bedrock-autoloader or roots/autoloader?
2. Should it live on as a mu-plugin or as a "normal" Composer package (discussed in Decouple logic from actual autoloader class filesystem location #5)?
3. Whom should we specify as authors?

Closes #1

[austinpray]

• Should it be named roots/bedrock-autoloader or roots/autoloader?

I suppose if we are doing things properly this is not necessarily coupled with bedrock specifically? It should really be like "plugin-autoloader" or something. cc @roots/everyone

• Should it live on as a mu-plugin or as a "normal" Composer package (discussed in Decouple logic from actual autoloader class filesystem location #5)?

Normal composer package as per @aaemnnosttv's recommendation. This is a library that one can use in their own file dropped in the mu-plugins directory.

• Whom should we specify as authors?

Added myself cause I'll be an admin on the composer repo

[retlehs]

@Foxaii is the original author, please add him to the list (at the 🔝)

[vinkla]

Closing due to inactivity.