Added support for important mark (!) analysis.

For example, an input of
GRPC_LAT_PROF 1316908107613.015869 0x7fd35f7fe700 { 205 (nil)
src/core/iomgr/tcp_posix.c 1
GRPC_LAT_PROF 1316908107614.015869 0x7fd35f7fe700 { 205 (nil)
src/core/iomgr/tcp_posix.c 2
GRPC_LAT_PROF 1316908107615.015869 0x7fd35f7fe700 ! 999999 (nil)
src/core/iomgr/tcp_posix.c 3
GRPC_LAT_PROF 1316908107616.015869 0x7fd35f7fe700 } 205 (nil)
src/core/iomgr/tcp_posix.c 4
GRPC_LAT_PROF 1316908107617.015869 0x7fd35f7fe700 ! 999999 (nil)
src/core/iomgr/tcp_posix.c 5
GRPC_LAT_PROF 1316908107618.015869 0x7fd35f7fe700 } 205 (nil)
src/core/iomgr/tcp_posix.c 6

results in

tag 50%/90%/95%/99% us
205 5.00/5.00/5.00/5.00

Important marks:
================
999999 @ src/core/iomgr/tcp_posix.c:3
205 { (src/core/iomgr/tcp_posix.c:1):        2.000 us
205 { (src/core/iomgr/tcp_posix.c:2):        1.000 us
205 } (src/core/iomgr/tcp_posix.c:4):       -1.000 us
205 } (src/core/iomgr/tcp_posix.c:6):       -3.000 us

999999 @ src/core/iomgr/tcp_posix.c:5
205 { (src/core/iomgr/tcp_posix.c:1):        4.000 us
205 } (src/core/iomgr/tcp_posix.c:6):       -1.000 us