fixes #2856

Person who merges PR is now expected to update the CHANGELOG.

Fixes #2815.

Many of the examples provided within documentation are using a single
dash for the command line arguments, but the telegraf executable
explicitly has two dashes.

There are also some inconsistencies with the ordering of the command
line argument examples.  I've ordered them so that the examples will
show: config, config-directory, input-filter, test

Using Keep-Alive would change the timing for already established
connections.  Previous to this commit, Keep-Alive worked only when using
a response_string_match due to failure to close the request body.

Hoping this will encourage more non-plugin contributions.

Mention that some metrics are available only at larger intervals than 5
minutes.  Update dead links to new locations and example config.

closes #1907

closes #2766