Add optional install path

This allows users to consume the install functionality but also install
to other areas instead of the managed `/opt` dir.

```bash
> ctr install --path /usr/local
```

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

install_opts.go

@@ -25,6 +25,8 @@ type InstallConfig struct {
 	Libs bool
 	// Replace will overwrite existing binaries or libs in the opt directory
 	Replace bool
+	// Path to install libs and binaries to
+	Path string
 }
 
 // WithInstallLibs installs libs from the image
@@ -36,3 +38,10 @@ func WithInstallLibs(c *InstallConfig) {
 func WithInstallReplace(c *InstallConfig) {
 	c.Replace = true
 }
+
+// WithInstallPath sets the optional install path
+func WithInstallPath(path string) InstallOpts {
+	return func(c *InstallConfig) {
+		c.Path = path
+	}
+}