Switch to shell script instead of postinstall

[karlhorky]

npm does not run postinstall on every package install (as @mike-clark-8192 mentioned in #65 (comment)).

This switches to a bash script as an alternative.

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Thinking about this more, it would be even nicer if typesync could take a package name as an argument:

typesync install <pkg name>

or just:

typesync <pkg name>

and then it would handle the installation, like typac does. Then there would be no need for a shell script.

[coveralls]

Coverage remained the same at 100.0% when pulling 0c25a8e on karlhorky:patch-1 into a623e61 on jeffijoe:master.