Add functionality to export settings

For the reproducer feature I need to be able to export and import the
current LLDB configuration. To realize this I've extended the existing
functionality to print settings. With the help of a new formatting
option, we can now write the settings and their values to a file
structured as regular commands.

Concretely the functionality works as follows:

  (lldb) settings export -f /path/to/file

This file contains a bunch of settings set commands, followed by the
setting's name and value.

  ...
  settings set use-external-editor false
  settings set use-color true
  settings set auto-one-line-summaries true
  settings set auto-indent true
  ...

You can import the settings again by either sourcing the file or using
the settings read command.

  (lldb) settings read -f /path/to/file

Differential revision: https://reviews.llvm.org/D52651

llvm-svn: 345346