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
I tried to be consistent with fugitive even here. The way I use this feature is as follows:
1. Get the status.
2. Select file, view its diff in the screen below.
3. Switch back to the status window above. Select different file, etc. 

I am interested to know how the existing view is used by folks.