add report parameter

[andreysaksonov]

fixes #16 and #18

[andreysaksonov]

usage example:

            <plugin>
                <groupId>com.github.ozsie</groupId>
                <artifactId>detekt-maven-plugin</artifactId>
                <version>${detekt.version}</version>
                <configuration>
                    <config>detekt.yml</config>
                    <report>xml:${project.basedir}/target/detekt.xml</report>
                </configuration>
            </plugin>

[codecov-io]

Codecov Report

Merging #19 into master will decrease coverage by 0.5%.
The diff coverage is 50%.

@@             Coverage Diff              @@
##             master      #19      +/-   ##
============================================
- Coverage     66.12%   65.62%   -0.51%     
  Complexity       12       12              
============================================
  Files             4        4              
  Lines            62       64       +2     
  Branches         18       19       +1     
============================================
+ Hits             41       42       +1     
  Misses            8        8              
- Partials         13       14       +1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/com/github/ozsie/DetektMojo.kt	64% <50%> (-0.59%)	4 <0> (ø)

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