Automatically sanitise parameter_meta strings

[multimeric]

If you try to:

from wdlgen import ParameterMeta
ParameterMeta(foo="\"bar\"").get_string()

The resulting WDL will look like this, which is a syntax error:

version 1.0

task Bedtools {
  parameter_meta {
    foo: ""bar""
  }
}

I think that quotes inside the string should be escaped using \".