cylc show: More user friendly pre-requisite labels

[oliver-sanders]

Closes #1346
Closes #2147?

Before

prerequisites (- => not satisfied):
  -     LABEL: a_colon_succeed = a.1 succeeded
  -     LABEL: b_colon_succeed = b.1 succeeded
  -     LABEL: c_colon_succeed = c.1 succeeded
  -     LABEL: d_colon_succeed = d.1 succeeded
  - CONDITION: a_colon_succeed&b_colon_succeed&(c_colon_succeed|d_colon_succeed)

After

prerequisites (- => not satisfied):
  - 0 & 1 & (2 | 3)
  - 	0 = a.1 succeeded
  - 	1 = b.1 succeeded
  - 	2 = c.1 succeeded
  - 	3 = d.1 succeeded

Adds --list-prereqs option which results in:

a.1 succeeded
b.1 succeeded
c.1 succeeded
d.1 succeeded

[hjoliver]

Good, thanks.