Render .md report for K003

This continues #160 (closed)

Basic version is done, improvements are needed.

• 1) Improve wording in comments above the table @dmius please see https://postgres-ai.slack.com/archives/CE6NYN64W/p1545975460008600

@dmius у К003 please fix lines before the table:

• there are 2 snapshots, so it's not correct to write "Snapshot start time". Better like in JSON: "Start:", "End:"

• "Period, seconds:" add the 4-th line "Period, age:"

• Remane the first column (Num) into # (escape in md.: \#)

• 2)

104734.48
ms
40.09
ms/sec
0.16
ms/call

is hard to read. We need to use 3 lines only, w/o additional unexpected line breaks.

• 3) this needs to be changed to regular format:

1.908795e+06

• 4) Instead of kcache_reads and kcache_writes we need kcache_read_blks and kcache_write_blks – similar to pg_stat_statements' columns. To get those, we need to do something like this:

# select 32768/current_setting('block_size')::numeric;
      ?column?
--------------------
 4.0000000000000000
(1 row)

/cc @Nastradamus