Fix for cleaned URL.Path

Fix for #854

Strip [] from IPv6 addresses for FastCGI.

Fixes #845.

systemd, README: Edit to account for the recent spike in reports

We have had three operators within a few days which ran into the same cause
and had not been able to figure out what went wrong.

addresses #833, #822

fix typo

Provides some more guidelines to operators on how to avoid running Caddy as root.

Introduces an user www-data, which really is a placeholder. Such an user with the same UID/GID combination is created on the most popular Linux distribution. I trust any operator can spot the difference to his/her distro and adjust the unit file.

User nobody is not used here to avoid two easy pitfalls: Such an user should not be able to access private keys (for TLS), and should not write private keys (we would do that with Letsencrypt).

The exemplary unit file for systemd is intentionally redundant at times, for
example dropping privileges which an unprivileged user "www-data" did not have
in the first place: To aid as fallback in case the file gets copied and an
operator setting UID to 0 (which reportedly happened in the past).

browse: Decorate external links with: noopener noreferrer

Setting these on external links prefents the target from manipulating this
page by "window.opener" with some widely deployed browsers.

Restart gracefully with in-process restart

Adding note on the rationale for Go 1.6

Markdown changes

* Overwrite proxy headers based on directive

Headers of the request sent by the proxy upstream can now be modified in
the following way:

Prefix header with `+`: Header will be added if it doesn't exist
otherwise, the values will be merge
Prefix header with `-': Header will be removed
No prefix: Header will be replaced with given value

* Add missing formating directive reported by go vet

* Overwrite up/down stream proxy headers

Add Up/DownStreamHeaders to UpstreamHost

Split `proxy_header` option in `proxy` directive into `header_upstream`
and `header_downstream`. By splitting into two, it makes it clear in
what direction the given headers must be applied.

`proxy_header` can still be used (to maintain backward compatability)
but its assumed to be `header_upstream`

Response headers received by the reverse proxy from the upstream host
are updated according the `header_downstream` rules.
The update occurs through a func given to the reverse proxy, which is
applied once a response is received.

Headers (for upstream and downstream) can now be modified in
the following way:

Prefix header with `+`: Header will be added if it doesn't exist
otherwise, the values will be merge
Prefix header with `-': Header will be removed
No prefix: Header will be replaced with given value

Updated branch with changes from master

* minor refactor to make intent clearer

* Make Up/Down stream headers naming consistent

* Fix error descriptions to be more clear

* Fix lint issue