Update toml library used for improved configuration ingestion

[glinton]

Adds support for:

• inline tables (resolves [[inputs.http]] error table `headers' is in conflict #4098 and provides a workaround for Panic decoding toml array when expecting string #3444)
• triple double quoted strings (resolves Use BurntSushi/toml for config file parsing #1598)

Caveats/outstanding issues:

• comments in within arrays are still not supported (3642). This fails:

  methods = [
    # comment
    "POST"
  ]

• illogical table boundaries (3444). This breaks:

[[outputs.http]]
    method = "POST"
    data_format = "json"
    url = "http://localhost"
    [outputs.http.headers]
       Authorization = "Token $TOKEN"
       Content-Type = "application/json"
    namepass = ["storage_usage_bytes"]
    taginclude = ["org_id"]

use inline tables:

[[outputs.http]]
    method = "POST"
    data_format = "json"
    url = "http://localhost:1234/"
    headers = { Authorization = "Token $TOKEN",Content-Type = "application/json"}
    namepass = ["storage_usage_bytes"]
    taginclude = ["org_id"]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.