New command to export a patch and allow existing log command to take opt...

...ions.

* HgExportPatch command takes patch name as input. If the env variable
  HG_EXPORT_PATCH_DIR is set, then the patch will be created under it.
  Otherwise, it will be created in the directory from which vim
  was launched.

* HgLog command takes options that can be passed to hg log command.
  E.g., the following command will list just 3 logs by user bob.
  :Hglog -u bob -l 3

Testing:
* Patch gets created under the right directory when env variable is set
  and not set.

* Hglog command honors -u and -l options. It also works when current
  file name is given as input --> :Hglog % -u bob -l 3
Little cosmetic remark: can you put the HgExportPatch in its own section? Here you added it to the "diff stuff" section.