add new Feature for response body contains String

[lmiz2]

new feature contains(or not contains)

config.json

... "requests":[ { "url":"http://localhost:5500", "requestType":"GET", "headers":{ }, "params":{ }, "checkEvery":5, "responseCode":200, "responseTime":800, "checkResponseBody" : { "checkType" : "contains", "matchFor" : "some String" } } ] ...

chechType can be "contains" or "notContains". If it is "contains", it is judged as an error if the response does not contain the string specified as "matchFor". else it is "notContains" it works as expected.