as requested in #1391

Advanced API

as suggested by @terrelln

Refactor examples to avoid code duplication.

Add buck build instructions to README.md

only applies in MT / async context now.

for naming consistency

so that it please both
compilers which warn for dead code after the switch
and
compilers which do not detect that all branches terminate.

and ZSTD_DCtx_setParameter()

advanced parameters compatible with ZSTD_decompressStream().

Update meson build and add Travis test for it

check availability of --color=never command on grep and egrep

before applying them.
Fixes #1436

[regression] Add dictionary support

* Install lz4 dep from Ubuntu repo
* Use curl instead of wget
* Use xenial for meson and make meson build first
* Unset Travis preset CC and CXX value for meson to use ccache
* Be more verbose with "set -x"