update webpack with parallel-webpack

[amrfarid140]

• Added helper functions to get list of all functions directories located in a directory
• Added helper function to map list of all functions directories located in a directory into Webpack entry object
• Modified webpack config file to generate a list of configuration objects for each entry found
• Renamed function files to be "index.ts"

** PR Checklist
Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/azure-seed/azure-functions-typescript/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features) No change in docs required

** PR Type
What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

** What is the current behavior?

Replacing addition of entry points for Webpack from a manual process to an automatic process by reading all the subfolders in the source directory. Also added parallel execution for each entry point which should speed up the process for medium & large sized funcitons app.

Issue Number: 53

** What is the new behavior?
Uses parallel-webpack to build multiple webpack configuration in parallel. Beneficial when multiple entry points exist.
** Does this PR introduce a breaking change?

[ ] Yes
[x] No

** Other information

[codecov]

Codecov Report

Merging #54 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #54   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          36     36           
  Branches        9      9           
=====================================
  Hits           36     36

Impacted Files	Coverage Δ
src/some-function/index.ts	100% <ø> (ø)

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