add metric option precomputed (#26)

[LGro]

I'm not sure why the metric validation in umap_.py:1308-1314 was commented out by you in da04e17, but in case you want to integrate this again, you would need to add the following:

elif metric == 'precomputed':
    pass

[pep8speaks]

Hello @LGro! Thanks for submitting the PR.

• In the file umap/umap_.py, following are the PEP8 issues :

Line 191:80: E501 line too long (80 > 79 characters)
Line 305:80: E501 line too long (80 > 79 characters)
Line 374:80: E501 line too long (80 > 79 characters)
Line 443:80: E501 line too long (80 > 79 characters)
Line 647:38: E231 missing whitespace after ','
Line 650:46: E231 missing whitespace after ','
Line 664:37: E127 continuation line over-indented for visual indent
Line 673:33: E128 continuation line under-indented for visual indent
Line 677:41: E128 continuation line under-indented for visual indent
Line 678:41: E128 continuation line under-indented for visual indent
Line 680:53: E128 continuation line under-indented for visual indent
Line 681:53: E128 continuation line under-indented for visual indent
Line 682:53: E128 continuation line under-indented for visual indent
Line 683:53: E128 continuation line under-indented for visual indent
Line 684:53: E128 continuation line under-indented for visual indent
Line 685:53: E128 continuation line under-indented for visual indent
Line 686:53: E128 continuation line under-indented for visual indent
Line 687:53: E128 continuation line under-indented for visual indent
Line 688:53: E128 continuation line under-indented for visual indent
Line 693:41: E128 continuation line under-indented for visual indent
Line 694:41: E128 continuation line under-indented for visual indent
Line 696:53: E128 continuation line under-indented for visual indent
Line 697:53: E128 continuation line under-indented for visual indent
Line 698:53: E128 continuation line under-indented for visual indent
Line 699:53: E128 continuation line under-indented for visual indent
Line 700:53: E128 continuation line under-indented for visual indent
Line 701:53: E128 continuation line under-indented for visual indent
Line 706:17: E128 continuation line under-indented for visual indent
Line 707:17: E128 continuation line under-indented for visual indent
Line 857:5: E741 ambiguous variable name 'I'
Line 875:80: E501 line too long (80 > 79 characters)
Line 1119:80: E501 line too long (80 > 79 characters)
Line 1120:9: E722 do not use bare except'
Line 1241:80: E501 line too long (80 > 79 characters)
Line 1267:80: E501 line too long (80 > 79 characters)

[LGro]

Sorry for falsely referencing #24 instead of #26 in the commit.

[lmcinnes]

Looks great! Thanks for taking the time to do this, it is greatly appreciated. If you have any further ideas for improvements please don't hesitate to propose them. I'll gladly accept more pull requests like this.