Add the ability to mount arbitrary folders in the VM

[cweagans]

First off - this PR won't break the existing config.

Here's a list of what changes:

• The documentroot for each site now refers to the path inside of the VM, so validation of the docroot path doesn't make sense anymore and has been removed. docroot validation still works for users that are still using synced_folder. Users that opt for shared_folders will be responsible for ensuring that their config works.
• If the synced_folder option is specified (as it has been in all previous configurations), the documentroot for each site is silently updated to reflect the default path in the vm (/var/www/html/ + site_docroot).
• If the shared_folders option is specified, the developer can mount any arbitrary folder in the VM at any path. This is considered to be more advanced functionality, so if the developer does something weird like mounting a folder at /etc or something, it's not our problem.

This is useful because I have all of my NBC code in ~/Documents/Code/NBCU, but I also want to be able to use the VM for non-NBC stuff (i.e. Drupal 8 contribution and such).

[cweagans]

Rebased this onto master, as I'm running this branch locally and want the other features too :)

[conortm]

Some nit-picky stuff from me. but overall :+1:!

[cweagans]

Alright, added checking for sites key and an example config file for multiple shared folders.

[conortm]

Looks good to me! Merging!