feat(conserve): add filter by --tag flag on conserve command

[aramalipoor]

Closes #6

• Introduce "-t|--tag" flag to narrow down targeted resources (using AWS.ResourceGroupsTaggingAPI)
• Introduce "prepareTags" step useful for filtering some resources by tag such as DynamoDB
• Add nock to disable all real network calls in tests

[codecov-commenter]

Codecov Report

Merging #31 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master       #31    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           18        18            
  Lines         1193      1343   +150     
  Branches       210       260    +50     
==========================================
+ Hits          1193      1343   +150     

Impacted Files	Coverage Δ
src/storage/local.storage.ts	100.00% <ø> (ø)
src/storage/s3.storage.ts	100.00% <ø> (ø)
src/storage/storage.resolver.ts	100.00% <ø> (ø)
src/tricks/trick.registry.ts	100.00% <ø> (ø)
src/base-command.ts	100.00% <100.00%> (ø)
src/commands/conserve.ts	100.00% <100.00%> (ø)
src/commands/restore.ts	100.00% <100.00%> (ø)
...cks/decrease-dynamodb-provisioned-rcu-wcu.trick.ts	100.00% <100.00%> (ø)
...rc/tricks/decrease-kinesis-streams-shards.trick.ts	100.00% <100.00%> (ø)
src/tricks/remove-nat-gateways.trick.ts	100.00% <100.00%> (ø)
... and 8 more

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