[Bug] jscoverage output files name is incorrect when use cli to convert ...

...from a souredir to destdir (in windows)

Example case:
I have a source dir:
```
──source
   ├─test.a.js
   ├─test.b.js
   └─test.c.js
```
then I use cli to convert  `/source` files to `/dest`
```js
jscoverage ./source ./dest/source
```

then I get dest files
```
──source
   ├─est.a.js
   ├─est.b.js
   └─est.c.js
```
Each filename in `dest` miss a char. Because you use `file.substr(source.length)` to get the file name,
but source is `./source` and file is `source/test.x.js`, so then sub length is wrong.

I use `file.split(path.sep).pop()` to get filename to fix