Allow to define content type/language for multiline strings

[bandesz]

Currently multiline strings can be defined using a pair of ''', e.g.

param = '''
  multiline
  string
'''

Allow to define the language just like in markdown, e.g.

param = '''json
  {
    "foo": "bar"
  }
'''

Also probably ''' should be change to three backticks, so it's convenient to use for people used to markdown.

The language definition should allow to do additional static analysis, e.g. validating embedded json, yaml or shell scripts.