`snap` should provide functionality to show files, which are included to installed snap (analog of `dpkg -L`)

[Norbert]

Steps to reproduce:
1. Install Ubuntu 14.04 LTS, 16.04 LTS, 17.10 or 18.04 LTS
2. Try to use `snap` to show files, which are included to installed snap (analog of `dpkg -L`)

Expected results:
* snap has functionality similar too `dpkg -L`

Actual results:
* snap do not have functionality similar to `dpkg -L`

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: snapd 2.29.4.2
ProcVersionSignature: Ubuntu 4.4.0-112.135-generic 4.4.98
Uname: Linux 4.4.0-112-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: amd64
CurrentDesktop: MATE
Date: Sat Feb 10 17:13:48 2018
InstallationDate: Installed on 2018-01-04 (36 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: snapd
UpgradeStatus: No upgrade log present (probably fresh install)

[John Lenton (chipaca)]

Note that, because snaps are self-contained, the answer to "list files in installed snap" is simply to find /snap/<snapname/current/.