Tidy up "Linking.." messages during compile

Linking message will only be printed the first time the link or copy
actually happens.

Note:

rebar_file_utils:symlink_or_copy will now return 'exists'
instead of ok, if it did nothing because the link exists.

Nothing was checking the return value yet, so seemed reasonable.

run edoc provider in docs provider

Bumped relx to 3.4.0 and erlware_commons to 0.15.0

handle global config file that can't be read

do not install profile deps of deps

add ssl cert validation, unless OTP version is too old

Making the file up to date

Escape paths and args in shell commands

Basic escaping is done only. Fancy hex sequences are not covered, but
this should otherwise take care of the most common issues.

Fixes #497

only look for top level apps and those directly under apps/ or lib/

Fixed erln8 link

keep plugins in path after compile provider run

Revert "don't add `{extra_src_dirs, ["test"]}` to `test` profile"

This reverts commit b06a6ecadd.

Global plugin bug

Fix missing 'cd' that depended on cwd changes

Add pr2relnotes tool to generate release notes

bug fix: was using outer LevelsAcc variable in inner foldl

Fix links in README

update travis to test on 18 and 17.5

pass failing upgrade case where levels are increased

Fix reporting of error:undef exceptions

The wide-cast catching of undef errors would make it so any provider
calling an undefined function would be reported as a missing 'do' for
the provider.

This patch inspects the stacktrace to know if `Provider:do/1` is indeed
the missing callback, and reports the rest properly.

upgrade erlware_commons, bbmustache and relx

Only have the port cd, not rebar3 as a whole

don't run unnecessary code if hooks are empty list