Adds support for triple-quoted possibly multi-line verbatim string literals

This is a simple parser addition that provides support for verbatim literals which could be useful
if writing expressions which usually need some escaping like file-paths, regular expressions, xml or longer text.

Example:

```Idris
filePath : String
filePath = """C:\Users\TestUser\Desktop\Test.xml"""

xmlTest : String
xmlTest = """
<person id="12">
   <name>Test User</name>
   <number>+2 3 4</number>
</person>
"""
```

Hopefully you find this useful too.