[MCOMPILER-442] Add "showLint" option

**[mike duigou](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=bondolo)** opened **[MCOMPILER-442](https://issues.apache.org/jira/browse/MCOMPILER-442?redirect=false)** and commented

Add a showLint configuration option to eliminate the need to use

```java

true

-Xlint:all

...

```

to configure the display of compiler lint messages. The advantage to a showLint configuration would be that it is more ammenable to inclusion in a parent or reactor project for inheritance in to sub-modules.

The \ element, if present would add the "-Xlint" option to the compiler command line and the contents of the element as the parameter. ie.

```java
all,-options,-path
```

would add:

```java
-Xlint:all,-options,-path
```

to the compiler command line.

---

**Affects:** 3.8.1

**Remote Links:**
- [plexus compiler Issue#115
](https://github.com/codehaus-plexus/plexus-compiler/issues/115)
- [plexus-compiler #131
](https://github.com/codehaus-plexus/plexus-compiler/pull/131)