Add systemd::escape function

Add a systemd escape function.

```puppet
systemd::escape('/foo/bar')
```
returns `-foo-bar`

while the path escape can be executed as

```puppet
systemd::escape('/foo/bar',true)
```

to return 'foo-bar'

functions/escape.pp

@@ -0,0 +1,22 @@
+function systemd::escape(String $input, Boolean $path = false) >> String {
+  # Escape method is defined
+  # https://www.freedesktop.org/software/systemd/man/systemd.unit.html
+  $_output = $input.map |$_i, $_token | {
+    case $_token {
+      '/': {
+        if $path and ($_i != 0 ) and $input[$_i-1] == '/' {
+          $_escaped = ''
+        } elsif $path and ($_i == 0 or $_i == ($input.length - 1) ) {
+          $_escaped = ''
+        } else {
+          $_escaped = '-'
+        }
+      }
+      ',': { $_escaped = '\x2c' }
+      default: { $_escaped = $_token }
+    }
+    $_escaped
+  }.join
+
+  return $_output
+}

spec/functions/escape_spec.rb

@@ -0,0 +1,17 @@
+require 'spec_helper'
+
+describe 'systemd::escape' do
+  context 'with path false' do
+    it { is_expected.to run.with_params('foo', false).and_return('foo') }
+    it { is_expected.to run.with_params('/foo/bar/', false).and_return('-foo-bar-') }
+    it { is_expected.to run.with_params('//foo//bar//', false).and_return('--foo--bar--') }
+    it { is_expected.to run.with_params('//foo:bar,foo_bar.//', false).and_return('--foo:bar\x2cfoo_bar.--') }
+  end
+
+  context 'with path true' do
+    it { is_expected.to run.with_params('foo', true).and_return('foo') }
+    it { is_expected.to run.with_params('/foo/bar/', true).and_return('foo-bar') }
+    it { is_expected.to run.with_params('//foo//bar//', true).and_return('foo-bar') }
+    it { is_expected.to run.with_params('//foo:bar,foo_bar.//', true).and_return('foo:bar\x2cfoo_bar.') }
+  end
+end