Bug with getting scenarios list

I have express js server which on get request start backstop test. Also i can send filter parameter. See example above
```
router.get('/', function(req, res, next) {
let filterVal = req.query.filter || "";
backstopDef = backstop('test', {
filter: filterVal
}) .then(function(val) {
res.json({ "answer": val})
})
.catch(function(reason) {
res.json({ "answer": 'Error ' + reason;})
})
});
```
And the problem is: When i make request with filter it filtered list of scenarios correct , but when i make next request without filter i got wrong list of scenarios.
Step to reproduce:
1. I have 2 test scenario
2. I make request and send filter parameter
3. I got correct result
4. I make request without filters
5. I got incorrect list of scenarious
I spend few hours yesterday to find why it happens and i`m sure that problem in createBitmaps.js ->decorateConfigForCapture
```
if (typeof config.args.config === 'object') {
configJSON = config.args.config;
} else {
configJSON = require(config.backstopConfigFileName); //<- Here we got not an copy of data but reference
}
```
And when we do it with filter
```
config.args.filter.split(',').forEach(function (filteredTest) {
each(configJSON.scenarios, function (scenario) {
if (regexTest(scenario.label, filteredTest)) {
scenarii.push(scenario);
}
});
});
configJSON.scenarios = scenarii;
```
we change reference value and on next call this function we got previously filtered scenarios.
I fix it just implemented copying content of config.backstopConfigFileName
from
```
configJSON = require(config.backstopConfigFileName);
```
To
```
configJSON = Object.assign({}, require(config.backstopConfigFileName));
```
It helps me to fix my issue and shouldn`t break anything else. It will be great if you can include this fix to library