Allow chained commands and setting env vars in commands

[kategengler]

• Add note to readme about needing to npm link ember-try

Allows:

ember try:one <scenario> --- FOO=5 ember test
ember try:one <scenario> --- 'ember test && echo 2' (note quotes)

The same goes for the command in the config file.

Fixes #94 and #87

[rwjblue]

Seems good, do we run appveyor (to confirm windows changes work)?

[kategengler]

@rwjblue See #97 re: appveyor. Plan to at least manually check windows for now.

[rwjblue]

Sounds good.

[kategengler]

Manually checked on windows: All the previous types of commands seem to work, but chained commands do not.

[kategengler]

On linux, npm link; npm link ember-try doesn't seem to be enough to get the ember try commands run to use the checked out version and not the version included by ember-cli.

[kellyselden]

FOO=5 ember test is invalid syntax on Windows. I've always used cross-env for setting env vars multi-platform. (here for example)

[kategengler]

@kellyselden cross-env probably works already, since it's a command. Part of this work was to allow setting environment variables in the command passed to ember-try. The later part, which should be possible, isn't working on windows (chained commands with && or ||), though it might be the same issue as what I'm seeing on linux: When all-commands.sh is run, the commands use the ember-try included with ember-cli, rather than the working copy.