Add a URL clustering method to reduce the cardinality

[dxsup]

Description

1. Add a new urlclustering package which contains a ClusteringMethod interface and two trivial implementations.
2. Add an option for choosing which URL clustering method is used. alphabet is used by default.

Related Issue

#267

How Has This Been Tested?

UnitTest

=== RUN   TestAlphabeticClusteringMethod_Clustering
--- PASS: TestAlphabeticClusteringMethod_Clustering (0.00s)
=== RUN   TestAlphabeticClusteringMethod_ClusteringBaseline
--- PASS: TestAlphabeticClusteringMethod_ClusteringBaseline (0.00s)
=== RUN   Test_isAlphabetical
=== RUN   Test_isAlphabetical/alphabetical
=== RUN   Test_isAlphabetical/non-alphabetical
--- PASS: Test_isAlphabetical (0.00s)
    --- PASS: Test_isAlphabetical/alphabetical (0.00s)
    --- PASS: Test_isAlphabetical/non-alphabetical (0.00s)
=== RUN   TestNoParamClusteringMethod_Clustering
--- PASS: TestNoParamClusteringMethod_Clustering (0.00s)
PASS

Benchmark

goos: darwin
goarch: amd64
pkg: github.com/Kindling-project/kindling/collector/pkg/urlclustering
cpu: Intel(R) Core(TM) i5-6267U CPU @ 2.90GHz
Benchmark_ClusteringBaseline
Benchmark_ClusteringBaseline-4   	  171873	      6979 ns/op
Benchmark_Clustering
Benchmark_Clustering-4           	  648603	      1991 ns/op
PASS

[hocktea214]

As an Alphabet version, it is able to merge some urls into the regular url.