Sort release tags *after* limiting

[salim-b]

Thanks a lot for implementing #13. Unfortunately, sortAscending = false is not very useful unless one also sets numtagstodisplay = -1, because the n releases since inception instead of since last release - 30 are displayed.

Currently, with defaults plus sortAscending = false:

> hvm use

  1) v0.63.2               2) v0.63.1               3) v0.63.0             
  4) v0.62.2               5) v0.62.1               6) v0.62.0             
  7) v0.61.0               8) v0.60.1               9) v0.60.0             
 10) v0.59.1              11) v0.59.0              12) v0.58.3             
 13) v0.58.2              14) v0.58.1              15) v0.58.0             
 16) v0.57.2              17) v0.57.1              18) v0.57.0             
 19) v0.56.3              20) v0.56.2              21) v0.56.1             
 22) v0.56.0              23) v0.55.6              24) v0.55.5             
 25) v0.55.4              26) v0.55.3              27) v0.55.2             
 28) v0.55.1              29) v0.55.0              30) v0.54.0

This PR should fix that.

Note that I have almost zero experience in Golang, so please bear with me if style or anything is not how you like it.