Allow multiple trees as input to Filter

[SparshithNR]

• Using fs-merger
  1. Allow filter developers to reduce mergeTree or funneling entire array of trees to one tree and then pass it to filter
  2. Allows input array of trees as input.
• input/output :
  1. Provides good ergonomics and avoids appending the this.outputPath
• Test:
  1. Added additional tests to make sure that Filter can take more than one tree

Example:

class Awk extends Filter {
  constructor(inputNode, search, replace, options) {
    options = options || {};
    super(inputNode, {
      annotation: options.annotation
    });
    this.search = search;
    this.replace = replace;
    this.extensions = ['txt'];
    this.targetExtension = 'txt';
  }
  processString(content, relativePath) {
    return content.replace(this.search, this.replace);
  };
}
module.exports = new Awk(['fixture','fixture_2'], 'test', 'real');

Since we are now allowing users to pass two paths/nodes. We avoid a uncessary merge/funnel. Before to achive the above result we had to below sinppet.

let fullTree = mergeTree(['fixture','fixture_2'];
module.exports = new Awk(fullTree, 'test', 'real');

[SparshithNR]

Input output facade is addressed in #187. We will address allowing multiple trees as input in this PR.

[SparshithNR]

Closing this as #188 is addressing this