MSVC 16.2 - Using 'vcunk' toolset

Hi,

Any chance to support 16.2 (preview 1)?

```
**********************************************************************
** Visual Studio 2019 Developer Command Prompt v16.2.0-pre.1.0
** Copyright (c) 2019 Microsoft Corporation
**********************************************************************

C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview>cd \vc\boost_1_70_0

C:\VC\boost_1_70_0>bootstrap.bat
Building Boost.Build engine

Failed to build Boost.Build engine.
Please consult bootstrap.log for further diagnostics.

bootstrap.log:
Found with vswhere Visual Studio Locator version 2.6.7+91f4c1d09e
[query version 2.1.1046.44959]
Found with vswhere Visual Studio Locator version 2.6.7+91f4c1d09e
[query version 2.1.1046.44959]
###
### Using 'vcunk' toolset.
###

C:\VC\boost_1_70_0\tools\build\src\engine>if exist bootstrap rd /S /Q bootstrap

C:\VC\boost_1_70_0\tools\build\src\engine>md bootstrap

C:\VC\boost_1_70_0\tools\build\src\engine>cl /nologo /RTC1 /Zi /MTd
/Fobootstrap/ /Fdbootstrap/ -DNT -DYYDEBUG -wd4996 kernel32.lib
advapi32.lib user32.lib /Febootstrap\jam0 command.c compile.c
constants.c debug.c execcmd.c execnt.c filent.c frames.c function.c
glob.c hash.c hdrmacro.c headers.c jam.c jambase.c jamgram.c lists.c
make.c make1.c object.c option.c output.c parse.c pathnt.c pathsys.c
regexp.c rules.c scan.c search.c subst.c timestamp.c variable.c
modules.c strings.c filesys.c builtins.c md5.c class.c cwd.c
w32_getreg.c native.c modules/set.c modules/path.c modules/regex.c
modules/property-set.c modules/sequence.c modules/order.c
command.c
compile.c
constants.c
debug.c
execcmd.c
execnt.c
filent.c
frames.c
function.c
glob.c
hash.c
hdrmacro.c
headers.c
jam.c
jambase.c
jamgram.c
lists.c
make.c
make1.c
object.c
Generating Code...
Compiling...
option.c
output.c
parse.c
pathnt.c
pathsys.c
regexp.c
rules.c
scan.c
search.c
subst.c
timestamp.c
variable.c
modules.c
strings.c
filesys.c
builtins.c
md5.c
class.c
cwd.c
w32_getreg.c
Generating Code...
Compiling...
native.c
set.c
path.c
regex.c
property-set.c
sequence.c
order.c
Generating Code...

C:\VC\boost_1_70_0\tools\build\src\engine>.\bootstrap\jam0 -f
build.jam --toolset=vcunk "--toolset-root=Visual Studio Locator
version 2.6.7+91f4c1d09e [query version
2.1.1046.44959]\Common7\Tools\..\..\VC\ " clean
###
### Unknown toolset: vcunk
###
### Known toolsets are: acc, borland, cc, como, clang, darwin, gcc,
gcc-nocygwin, intel-darwin, intel-linux, intel-win32, kcc, kylix,
metrowerks, mipspro, msvc, qcc, pathscale, pgi, sun, sunpro, tru64cxx,
vacpp, xlcpp, vc7, vc8, vc9, vc10, vc11, vc12, vc14, vc141, vc142,
vmsdecc
###

C:\VC\boost_1_70_0\tools\build\src\engine>.\bootstrap\jam0 -f
build.jam --toolset=vcunk "--toolset-root=Visual Studio Locator
version 2.6.7+91f4c1d09e [query version
2.1.1046.44959]\Common7\Tools\..\..\VC\ "
###
### Unknown toolset: vcunk
###
### Known toolsets are: acc, borland, cc, como, clang, darwin, gcc,
gcc-nocygwin, intel-darwin, intel-linux, intel-win32, kcc, kylix,
metrowerks, mipspro, msvc, qcc, pathscale, pgi, sun, sunpro, tru64cxx,
vacpp, xlcpp, vc7, vc8, vc9, vc10, vc11, vc12, vc14, vc141, vc142,
vmsdecc
###

C:\VC\boost_1_70_0\tools\build\src\engine>exit /b 1
```