Support compiling multiple files at once

[adamconnelly]

To keep things simple to begin with, the compiler only supports compiling a single Thrift file. We should support compiling multiple files at once so an entire directory tree can be compiled with a single command.

Scenario 1 - specify multiple input files

thrift-compiler --input UserType.thrift --input UserService.thrift --output-directory output

Scenario 2 - use a wildcard

thrift-compiler --input *.thrift --output-directory output

Scenario 3 - use a glob pattern

thrift-compiler --input **/*.thrift --output-directory output