Add -s/--source option support (MI)

Summary:
This patch adds -s/--source option to execute source file with prepared command.
For example:
```
$ cat start_script 
target create ~/p/hello
process launch -s
continue
$ bin/lldb-mi -s start_script 
(gdb)
target create ~/p/hello
Current executable set to '~/p/hello' (x86_64).
^done
(gdb)
process launch -s
=shlibs-added,shlib-info=[num="1",name="hello",dyld-addr="-",reason="dyld",path="/Users/IliaK/p/hello",loaded_addr="-",dsym-objpath="/Users/IliaK/p/hello.dSYM/Contents/Resources/DWARF/hello"]
Process 33289 launched: '/Users/IliaK/p/hello' (x86_64)
^done
(gdb)
continue
=thread-created,id="1",group-id="i1"
=thread-selected,id="1"
(gdb)
Process 33289 resuming
Process 33289 exited with status = 0 (0x00000000) 
^done
```

Test Plan: ./dotest.py -v --executable $BUILDDIR/bin/lldb tools/lldb-mi/

Reviewers: abidh

Reviewed By: abidh

Subscribers: lldb-commits, abidh

Differential Revision: http://reviews.llvm.org/D9278

llvm-svn: 236703