Add a property to EmbeddedCDXServerIndex for customizing default statuscode filter.

added baseStatusRegexp property to EmbeddedCDXServerIndex. 
this property takes a regular expression for status code included in capture search query. For example:
```
<property name="baseStatusRegexp" value="[23].."/>
```
will include captures with 2xx and 3xx statuscode only. leading "!" negates match condition:
```
<property name="baseStatusRegexp" value="!(500|502)"/>
```
will exclude captures with 500 or 502 status.
value null, empty string or "!" will disable filtering by statuscode.