Provide extensions to safely escape input strings

[kislyuk]

Per openwdl/wdl#443, the spec does not provide a straightforward way to safely interpolate strings in command blocks. This issue is about providing this functionality on an extension basis (with the hope of suggesting it for standardization in the future). Specifically, we can use a runtime configuration flag or plugin to control the behavior of quote() and squote(), making them work on scalars and escape internal quotes in the string using an escape character (defaulting to \).