Add confidence intervals to polyA estimator

[mparker2]

Adds 95% confidence intervals to read rate and poly(A) length estimates by performing bootstrapped resampling of kmer durations. By default this is switched off and the output should be identical to the current version. When switched on using --ci flag an extra 4 columns are added to the output. Default is to perform 100 bootstraps but this can be varied with --nboot option.