Add handling for custom partition table offsets

[thetek42]

• Add a --partition-table-offset flag that corresponds to esp-idf's CONFIG_PARTITION_TABLE_OFFSET
  • If the flag is specified, the partition table will be flashed to the given location.
  • If the flag is omitted, the partition table location will be determined automatically from the partition table by subtracting 0x1000 from the first partition, which should be the most common case.
• The location of the application is now automatically determined from the partition table instead of the hard-coded 0x10000.

Closes #458

[jessebraham]

LGTM, thanks!